Add derives to support custom types

[mehcode]

Supersedes #97

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@Freax13 I've rebased this and did some work. Part of the fall-out is we've since added support for TEXT mode in Postgres. I made a tiny parser for TEXT mode records.

• Add PgRecordDecoder and PgRecordEncoder to try and encapsulate some logic a little tighter.

• Implement Type and Decode (but not Encode because Postgres doesn't support it) for tuples (up to an arity of 9).

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• Update struct derives to use new encoder/decoder types
• Fix derives, Encode is now parametric over a buffer type and Decode is parametric over the raw value
  • transparent
  • records ( structs )
  • weak enums
  • strong enums

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I plan on finishing this in a couple days. If you get time before then and would like to assist, I'd appreciate it. If not, no worries.