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

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

Implement code generator plugins.  The Cap'n Proto compiler, when given -o foo, invokes the binary capnp-foo and passes the schema, in Cap'n Proto format, to its stdin.  The binary is executed with the current working directory set to the desired output directory, so all it has to do is write the files.