Refactor argument-handling to use a struct (#103)

## Summary:
In this commit I refactor the argument-generation logic to move most of
the code out of the template and into the type-generator.  This logic
predates #51, and I didn't think to update it there, but I think it
benefits from similar treatment, for similar reasons.

Specifically, the main change is to treat variables as another struct
type we can generate, rather than handling them inline as a
`map[string]interface{}`.  Users still pass them the same way, but
instead of putting them into a `map[string]interface{}` and JSONifying
that, we generate a struct and put them there.

This turns out to simplify things quite a lot, because we already have a
lot of code to generate types.  Notably, the omitempty code goes from a
dozen lines to basically two, and fixes a bug (#43) in the process,
because now that we have a struct, `json.Marshal` will do our work for
us! (And, once we have syntax for it (#14), we'll be able to handle
field-level omitempty basically for free.)  More importantly, it will
simplify custom marshalers (#38, forthcoming) significantly, since we do
all that logic at the containing-struct level, but will need to apply it
to arguments.

It does require two breaking changes:

1. For folks implementing the `graphql.Client` API (rather than just
   calling `NewClient`): we now pass them variables as an `interface{}`
   rather than a `map[string]interface{}`.  For most callers, including
   Khan/webapp, this is basically a one-line change to the signature of
   their `MakeRequest`, and it should be a lot more future-proof.
2. genqlient's handling of the `omitempty` option has changed to match
   that of `encoding/json`, in particular it now never considers structs
   "empty".  The difference was never intentional (I just didn't realize
   that behavior of `encoding/json`); arguably our behavior was more
   useful but I think that's outweighed by the value of consistency with
   `encoding/json` as well as the simpler and more correct
   implementation (fixing #43 is actually quite nontrivial otherwise).
   Once we have custom unmarshaler support (#38), users will be able to
   map a zero value to JSON null if they wish, which is mostly if not
   entirely equivalent for GraphQL's purposes.

Issue: https://github.com/Khan/genqlient/issues/38
Issue: https://github.com/Khan/genqlient/issues/43

## Test plan:
make check

Author: benjaminjkraft

Reviewers: StevenACoffman, dnerdy, aberkan, jvoll, mahtabsabet, MiguelCastillo

Required Reviewers: 

Approved By: StevenACoffman, dnerdy

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

Pull Request URL: https://github.com/Khan/genqlient/pull/103

generate/generate.go

@@ -55,8 +55,11 @@ type operation struct {
 	Doc string `json:"-"`
 	// The body of the operation to send.
 	Body string `json:"query"`
-	// The arguments to the operation.
-	Args []argument `json:"-"`
+	// The type of the argument to the operation, which we use both internally
+	// and to construct the arguments.  We do it this way so we can use the
+	// machinery we have for handling (and, specifically, json-marshaling)
+	// types.
+	Input *goStructType `json:"-"`
 	// The type-name for the operation's response type.
 	ResponseName string `json:"-"`
 	// The original filename from which we got this query.
@@ -69,14 +72,6 @@ type exportedOperations struct {
 	Operations []*operation `json:"operations"`
 }
 
-type argument struct {
-	GoName      string
-	GoType      string
-	GraphQLName string
-	IsSlice     bool
-	Options     *genqlientDirective
-}
-
 func newGenerator(
 	config *Config,
 	schema *ast.Schema,
@@ -125,29 +120,6 @@ func (g *generator) WriteTypes(w io.Writer) error {
 	return nil
 }
 
-func (g *generator) getArgument(
-	arg *ast.VariableDefinition,
-	operationDirective *genqlientDirective,
-) (argument, error) {
-	_, directive, err := g.parsePrecedingComment(arg, arg.Position)
-	if err != nil {
-		return argument{}, err
-	}
-
-	graphQLName := arg.Variable
-	goTyp, err := g.convertInputType(arg.Type, directive, operationDirective)
-	if err != nil {
-		return argument{}, err
-	}
-	return argument{
-		GraphQLName: graphQLName,
-		GoName:      lowerFirst(graphQLName),
-		GoType:      goTyp.Reference(),
-		IsSlice:     arg.Type.Elem != nil,
-		Options:     operationDirective.merge(directive),
-	}, nil
-}
-
 // usedFragmentNames returns the named-fragments used by (i.e. spread into)
 // this operation.
 func (g *generator) usedFragments(op *ast.OperationDefinition) ast.FragmentDefinitionList {
@@ -277,12 +249,9 @@ func (g *generator) addOperation(op *ast.OperationDefinition) error {
 		return err
 	}
 
-	args := make([]argument, len(op.VariableDefinitions))
-	for i, arg := range op.VariableDefinitions {
-		args[i], err = g.getArgument(arg, directive)
-		if err != nil {
-			return err
-		}
+	inputType, err := g.convertArguments(op, directive)
+	if err != nil {
+		return err
 	}
 
 	responseType, err := g.convertOperation(op, directive)
@@ -313,7 +282,7 @@ func (g *generator) addOperation(op *ast.OperationDefinition) error {
 		// rather than in the template so exported operations will match
 		// *exactly* what we send to the server.
 		Body:           "\n" + builder.String(),
-		Args:           args,
+		Input:          inputType,
 		ResponseName:   responseType.Reference(),
 		SourceFilename: sourceFilename,
 		Config:         g.Config, // for the convenience of the template