Parser consolidation follow-ups

[johnnyshields]

These are follow-ups from PR #759 to resolve in the future:

• Remove SignedDocumentWrapper, consolidate with Response class
• Memoize SignedDocumentInfo methods
• Use Memoizable mixin everywhere
• Review performance and overall flow
• Do some sort of response loader / response validator class
• remove .is_a?(TrueClass) from tests