Multiple schema-files are now supported as of #134, but the support was
a bit different from how we did multiple operation-files. Before anyone
starts to depend on the ways the syntaxes differ, let's just make them
the same. Since it's easy, I also added support for having just a
single operations-file.
I also realized while writing this that the type-change is technically
breaking (if you call from Go), so documented it as such. I think
this is unlikely to affect many people.
Test plan: make check
