Add a new option to treat an interface like an object (#97)

## Summary:
The basic idea here is if you only request interface fields (no
fragments) you may not care about the concrete type, and so we could
just generate a struct as if it were an object.  I don't think it's a
good idea to do that by default, because then if you later add a
fragment all your code totally changes, but it's quite reasonable as an
option!

Most of the code involved is just wiring and validation; the
core implementation is literally just: treat it like an object.

Issue: https://github.com/Khan/genqlient/issues/85

## Test plan:
make check


Author: benjaminjkraft

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

Required Reviewers: 

Approved By: StevenACoffman

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/97

generate/testdata/snapshots/TestGenerate-StructOption.graphql-StructOption.graphql.json

@@ -0,0 +1,9 @@
+{
+  "operations": [
+    {
+      "operationName": "StructOption",
+      "query": "\nquery StructOption {\n\troot {\n\t\tid\n\t\tchildren {\n\t\t\t__typename\n\t\t\tid\n\t\t\tparent {\n\t\t\t\tid\n\t\t\t\tchildren {\n\t\t\t\t\t__typename\n\t\t\t\t\tid\n\t\t\t\t}\n\t\t\t\tinterfaceChildren: children {\n\t\t\t\t\t__typename\n\t\t\t\t\tid\n\t\t\t\t\t... VideoFields\n\t\t\t\t}\n\t\t\t}\n\t\t}\n\t}\n\tuser {\n\t\troles\n\t}\n}\nfragment VideoFields on Video {\n\tduration\n}\n",
+      "sourceLocation": "testdata/queries/StructOption.graphql"
+    }
+  ]
+}