Complete querygen implementation

[obmarg]

#12 introduced querygen, but skipped over a few features:

• Query fragments
• Union types
• Interfaces
• Deduplication of types (if a type appears at multiple places it'll be emitted multiple times)
• Handling multiple queries
• There's a number of unwraps that should return actual errors
• Possibly other bits (do some exploration and see what else is missing)

[obmarg]

Deduplication of types & Handling multiple queries dealt with in #141

[obmarg]

Split this out into a couple of different issues:

• union types & interfaces in Support for interfaces & union types in querygen #104
• Query fragments in Support query fragments in generator #146

Going to close this one.