code generation to support concurrent resolution / fetches

[willscott]

Currently, the codegen for resolve over struct fields, map keys, lists, and unions does the work of resolution to fetch needed data and return the expected object.

Per https://github.com/graphql-go/graphql/blob/master/examples/concurrent-resolvers/main.go#L33-L35
if they immediately return a function that then does the work, the graphql loader framework will be able to process resolutions in parallel, which would be a nice speed boost.