Improve const-correctness of readers and builders.  We enforce transitive constness, meaning you should not be able to construct a mutable Builder from a const Builder (you can always convert them to Readers instead).

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.