## Summary:
In #52, I added support for interface types, but with the simplifying
restriction (among others) that the user must request the field
`__typename`. In this commit, I remove this restriction.
The basic idea is simple: we preprocess the query to add `__typename`.
The implementation isn't much more complicated! Although it required
some new wiring in a few places.
Issue: https://github.com/Khan/genqlient/issues/8
## Test plan:
make check
Author: benjaminjkraft
Reviewers: dnerdy, aberkan, MiguelCastillo
Required Reviewers:
Approved by: dnerdy
Checks: ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Test (1.13), ⌛ Lint, ⌛ Test (1.17), ⌛ Test (1.16), ⌛ Test (1.15), ⌛ Test (1.14), ⌛ Test (1.13), ⌛ Lint
Pull request URL: https://github.com/Khan/genqlient/pull/56
