Add support for abstract-typed named fragments (#79)

## Summary:
In previous commits I added support to genqlient for interfaces,
inline fragments, and, most recently, named fragments of concrete
(object) type.  This leaves only named fragments of interface type!
Like other named fragments, these are useful for code-sharing,
especially if you want some code that can handle the same fields of
several different types.

As seems to be inevitable with genqlient, this was mostly pretty
straightforward, although there turned out to be surprisingly many
places we needed to add some handling; almost anywhere that touches
interfaces *or* named fragments needed some updates.  But it's all
hopefully fairly clear code.

As a part of this change I made three semi-related improvements:
1. I refactored the handling of descriptions (i.e. GoDoc), because it
   was getting more and more confusing and duplicative.  I'm still not
   sure how much of it it makes sense to inline vs. separate, but I
   think this is better than it was.  This resulted in some minor
   changes to descriptions, generally in the direction of making things
   more consistent.
2. I bumped the minimum Go version to 1.14 so we can guarantee support
   for duplicate interface methods.  These are useful for
   abstract-in-absstract spreads; we generate an interface for the
   fragment, and (if the fragment-type implements the scope-type) we
   embed it into the interface we generate for its spread-context, and
   if the two have a duplicated field we thus duplicate the method.  It
   wouldn't be impossible to support this on 1.13 (maybe just by
   omitting said embed) but it didn't seem worth it.  This also removes
   a few special-cases in tests.
3. I added a bunch of code to better format syntax errors in the
   generated code (which we see from `gofmt`).  This is mostly just an
   internal improvement; I wrote it because I got annoyed while hunting
   down a few such errors..

Fixes, at last, #8.

Issue: https://github.com/Khan/genqlient/issues/8

## Test plan:
make check


Author: benjaminjkraft

Reviewers: dnerdy, benjaminjkraft, aberkan, MiguelCastillo

Required Reviewers: 

Approved By: dnerdy

Checks: ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/79

generate/types.go

@@ -101,14 +101,10 @@ func (typ *goEnumType) Reference() string { return typ.GoName }
 // goStructType represents a Go struct type used to represent a GraphQL object
 // or input-object type.
 type goStructType struct {
-	GoName      string
-	Description string
-	GraphQLName string
-	Fields      []*goStructField
-	// Incomplete is set if this type contains only certain fields of the
-	// corresponding GraphQL type (i.e. those selected by the operation) in
-	// which case we put a note in the doc-comment saying as much.
-	Incomplete bool
+	GoName  string
+	Fields  []*goStructField
+	IsInput bool
+	descriptionInfo
 }
 
 type goStructField struct {
@@ -133,24 +129,7 @@ func (field *goStructField) IsEmbedded() bool {
 }
 
 func (typ *goStructType) WriteDefinition(w io.Writer, g *generator) error {
-	description := typ.Description
-	if typ.Incomplete {
-		// For types where we only have some fields, note that, along with
-		// the GraphQL documentation (if any).  We don't want to just use
-		// the GraphQL documentation, since it may refer to fields we
-		// haven't selected, say.
-		prefix := fmt.Sprintf(
-			"%v includes the requested fields of the GraphQL type %v.",
-			typ.GoName, typ.GraphQLName)
-		if description != "" {
-			description = fmt.Sprintf(
-				"%v\nThe GraphQL type's documentation follows.\n\n%v",
-				prefix, description)
-		} else {
-			description = prefix
-		}
-	}
-	writeDescription(w, description)
+	writeDescription(w, structDescription(typ))
 
 	needUnmarshaler := false
 	fmt.Fprintf(w, "type %s struct {\n", typ.GoName)
@@ -211,37 +190,27 @@ func (typ *goStructType) Reference() string { return typ.GoName }
 // goInterfaceType represents a Go interface type, used to represent a GraphQL
 // interface or union type.
 type goInterfaceType struct {
-	GoName      string
-	Description string
-	GraphQLName string
+	GoName string
 	// Fields shared by all the interface's implementations;
 	// we'll generate getter methods for each.
 	SharedFields    []*goStructField
 	Implementations []*goStructType
+	descriptionInfo
 }
 
 func (typ *goInterfaceType) WriteDefinition(w io.Writer, g *generator) error {
-	goTypeNames := make([]string, len(typ.Implementations))
-	for i, impl := range typ.Implementations {
-		goTypeNames[i] = impl.Reference()
-	}
-
-	description := fmt.Sprintf(
-		"%v includes the requested fields of the GraphQL interface %v.\n\n"+
-			"%v is implemented by the following types:\n\t%v",
-		typ.GoName, typ.GraphQLName, typ.GoName, strings.Join(goTypeNames, "\n\t"))
-	if description != "" {
-		description = fmt.Sprintf(
-			"%v\n\nThe GraphQL type's documentation follows.\n\n%v",
-			description, typ.Description)
-	}
-	writeDescription(w, description)
+	writeDescription(w, interfaceDescription(typ))
 
 	// Write the interface.
 	fmt.Fprintf(w, "type %s interface {\n", typ.GoName)
 	implementsMethodName := fmt.Sprintf("implementsGraphQLInterface%v", typ.GoName)
 	fmt.Fprintf(w, "\t%s()\n", implementsMethodName)
 	for _, sharedField := range typ.SharedFields {
+		if sharedField.GoName == "" { // embedded type
+			fmt.Fprintf(w, "\t%s\n", sharedField.GoType.Reference())
+			continue
+		}
+
 		methodName := "Get" + sharedField.GoName
 		description := ""
 		if sharedField.GraphQLName == "__typename" {
@@ -269,6 +238,9 @@ func (typ *goInterfaceType) WriteDefinition(w io.Writer, g *generator) error {
 		fmt.Fprintf(w, "func (v *%s) %s() {}\n",
 			impl.Reference(), implementsMethodName)
 		for _, sharedField := range typ.SharedFields {
+			if sharedField.GoName == "" { // embedded
+				continue // no method needed
+			}
 			description := fmt.Sprintf(
 				"Get%s is a part of, and documented with, the interface %s.",
 				sharedField.GoName, typ.GoName)