TODO:
- Rename Guarded to Bounded?
- Consider bounded array (where size and indexes are bounded quantities).
- Implement non-CAPNP_DEBUG_TYPES fallback.
  - Don't allow casting kj::maxValue to bounded type, this won't work right when not using debug types!
- Verify that this change doesn't hurt performance.

json.capnp.{h,c++} must have been generated using an earlier version of
the code generator, or an earlier version of capnpc-c++.

Cloning capnproto/master, building and installing, then running
regenerate-bootstraps.sh results in this diff.