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.

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

Mass rename 'reference' -> 'pointer'.  Been meaning to do this for a while -- all the documentation talks about 'pointers', not 'references'.

Revamp dynamic struct getters/setters to not distinguish unions.  Add converience versions that take string names.

Dynamic API mostly done...  not tested.  This commit actually starts embedding the schema data in the generated code.