Track botocore order of shape fields in generator

[endgame]

In #862 the generator was changed to emit request/response fields in sorted order. We could switch the generator from aeson to waargonaut, which uses a JSON AST that tracks such things.

[endgame]

The new parser in aeson >= 2.1.2.0, Data.Aeson.Decoding.Tokens is probably easier to use than waargonaut will be, as its hw-simd-json dependency doesn't build right now.