Never use marshaled types in a union struct

[AArnott]

• Force unmarshaled fields in union structs. Fixes Could not load type "that is incorrectly aligned or overlapped by a non-object field" #292
• Propagate generation context
• Recognize that certain delegates are never declared as managed
• Fix type ref to type def conversion in light of multiple platform definitions
• Avoid marshalling in disallowed contexts
• Defend against infinite recursion in certian error conditions
• Small touch-ups
• Never emit an array when marshaling is not permitted