Gracefully handle the case where the schema contains types from a newer version of Cap'n Proto that we don't know about yet.

Don't trust trusted messages so much.  Just trust that the pointers are valid and in-bounds, but don't assume the data matches the schema.  This makes it easier to safely create a 'trusted' message: just copy any message into a MessageBuilder, and if it ends up all in one segment, it's safe to use.

totalSizeInWords() method on generated struct readers/builders returns total size of the struct, not counting far pointer overhead.  Useful for allocating space for a flat copy.  Also a couple misc tweaks.

Implement ability to set() struct and list fields by deep-copying from a corresponding Reader.  This copy requires no schema knowledge.