Add support for specifying type-names, and conflict-detection (#94)

## Summary:
In this commit I add two related features to genqlient:
conflict-detection to avoid generating two distinct types with the same
name, and an option to specify the type-name genqlient should use for
some type.

The conflict-detection was pretty simple once I realized I had already
written all the code to do it in #70.  There was a bunch of wiring,
since we now need to keep track of the GraphQL type/selection-set that
each type corresponds to, but it was pretty straightforward.  This
allows us to:
- detect and reject if you have really sneaky type-names (there are some
  examples documented in `names.go`)
- more clearly crash if genqlient accidentally generates two conflicting
  types, and
- avoid stack-overflow when handing recursive (input) types (although
  sadly the poor support for options on input types (#14) makes them
  difficult to use in many cases; you really need to be able to set
  `pointer: true`)

And with that all set up, the type-naming was also easy!  (It doesn't
have to get into the core of the type-generator, just plug in where we
choose names.  The desire for conflict detection was the main reason I
hadn't set it up already.)  Note that the existing limitation of #70 that
the fields have to be in exactly the same order remains (and is now
documented as #93); it's not deeply hard to fix but it's surprisingly
much work.

Issue: https://github.com/Khan/genqlient/issues/60
Issue: https://github.com/Khan/genqlient/issues/12

## Test plan:
make check


Author: benjaminjkraft

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

Required Reviewers: 

Approved By: StevenACoffman, jvoll

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

Pull Request URL: https://github.com/Khan/genqlient/pull/94

generate/testdata/snapshots/TestGenerate-Recursion.graphql-Recursion.graphql.go

@@ -0,0 +1,71 @@
+package test
+
+// Code generated by github.com/Khan/genqlient, DO NOT EDIT.
+
+import (
+	"github.com/Khan/genqlient/graphql"
+	"github.com/Khan/genqlient/internal/testutil"
+)
+
+// RecursionRecurRecursive includes the requested fields of the GraphQL type Recursive.
+type RecursionRecurRecursive struct {
+	Rec RecursionRecurRecursiveRecRecursive `json:"rec"`
+}
+
+// RecursionRecurRecursiveRecRecursive includes the requested fields of the GraphQL type Recursive.
+type RecursionRecurRecursiveRecRecursive struct {
+	Rec RecursionRecurRecursiveRecRecursiveRecRecursive `json:"rec"`
+}
+
+// RecursionRecurRecursiveRecRecursiveRecRecursive includes the requested fields of the GraphQL type Recursive.
+type RecursionRecurRecursiveRecRecursiveRecRecursive struct {
+	Rec RecursionRecurRecursiveRecRecursiveRecRecursiveRecRecursive `json:"rec"`
+}
+
+// RecursionRecurRecursiveRecRecursiveRecRecursiveRecRecursive includes the requested fields of the GraphQL type Recursive.
+type RecursionRecurRecursiveRecRecursiveRecRecursiveRecRecursive struct {
+	Id testutil.ID `json:"id"`
+}
+
+// RecursionResponse is returned by Recursion on success.
+type RecursionResponse struct {
+	Recur RecursionRecurRecursive `json:"recur"`
+}
+
+type RecursiveInput struct {
+	Rec []RecursiveInput `json:"rec"`
+}
+
+func Recursion(
+	client graphql.Client,
+	input RecursiveInput,
+) (*RecursionResponse, error) {
+	variables := map[string]interface{}{
+		"input": input,
+	}
+
+	var err error
+
+	var retval RecursionResponse
+	err = client.MakeRequest(
+		nil,
+		"Recursion",
+		`
+query Recursion ($input: RecursiveInput!) {
+	recur(input: $input) {
+		rec {
+			rec {
+				rec {
+					id
+				}
+			}
+		}
+	}
+}
+`,
+		&retval,
+		variables,
+	)
+	return &retval, err
+}
+