Expose the graphql operation as a constant in the generated genqlient file (#238)

This allows client code to see the operation (query or mutation) exactly
as genqlient sends it over the wire. This data was already available in
the generated safelist.json file, but now it's easily available from Go
code as well.
    
Fixes #236

I have:
- [x] Written a clear PR title and description (above)
- [x] Signed the [Khan Academy CLA](https://www.khanacademy.org/r/cla)
- [x] Added tests covering my changes, if applicable
- [x] Included a link to the issue fixed, if applicable
- [x] Included documentation, for new features
- [x] Added an entry to the changelog

docs/CHANGELOG.md

@@ -25,6 +25,7 @@ When releasing a new version:
 ### New features:
 
 - You can now bind all types from a package in `genqlient.yaml` using the new `package_bindings` option.
+- The graphql operation (query or mutation) as sent over the wire is now exposed via a constant in the generated genqlient .go file.
 
 ### Bug fixes: