Compile the generator as part of the build steps.

This is the first step toward getting the capnproto 'header' .capnp
files (c++.capnp, schema.capnp, etc) to be available in the right
place. Next is to generate those as well
Small fixes to type-conversion, plus addition of AnyPointer
Generate code for the capnp builtin schemas (schema.capnp, etc) at build time.
Update the README to provide usage and examples
Update to latest capnp_generic_gen
Fix submodule locations