Feature/all of support

[AlexanderMann]

Motivation

#14

This pr seeks to add full support for allOf to Target-Postgres.

This is done in a couple ways:

• validation of all records against the raw schema (not our simplified one)
  • we assume that impossible schemas will simply error violently here and thus we don't need to check for them later on
• simplification of the schema does a few things for allOf:
  • prefer datetimes
  • prefer literals
  • nullables break ties
  • merge all objects together
  • recurse (includes support for $refs in allOf!)