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/queries/Recursion.graphql

@@ -0,0 +1,6 @@
+query Recursion($input: RecursiveInput!) {
+  recur(input: $input) {
+    # (sadly, or happily, GraphQL doesn't let us recur infinitely here)
+    rec { rec { rec { id } } }
+  }
+}

generate/testdata/queries/TypeNames.graphql

@@ -0,0 +1,10 @@
+# @genqlient(typename: "Resp")
+query TypeNames {
+  # @genqlient(typename: "User")
+  user { id name }
+  # @genqlient(typename: "Item")
+  randomItem { id name }
+  # (ok to reuse the name as long as they match)
+  # @genqlient(typename: "User")
+  users { id name }
+}

generate/testdata/queries/schema.graphql

@@ -131,6 +131,15 @@ type Topic implements Content {
   schoolGrade: String
 }
 
+input RecursiveInput {
+  rec: [RecursiveInput]
+}
+
+type Recursive {
+  id: ID!
+  rec: Recursive
+}
+
 """Query's description is probably ignored by almost all callers."""
 type Query {
   """user looks up a user by some stuff.
@@ -140,7 +149,7 @@ type Query {
   """
   user(query: UserQueryInput): User
 
-  users(query: [UserQueryInput]): User
+  users(query: [UserQueryInput]): [User]
 
   """usersWithRole looks a user up by role."""
   usersWithRole(role: Role!): [User!]!
@@ -153,6 +162,7 @@ type Query {
   getComplexJunk: ComplexJunk
   listOfListsOfLists: [[[String!]!]!]!
   listOfListsOfListsOfContent: [[[Content!]!]!]!
+  recur(input: RecursiveInput!): Recursive
 }
 
 type Mutation {