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.

https://github.com/sandstorm-io/capnproto/issues/334

Add thisCap() method to capability servers which returns a capability to itself. Remove CapabilityServerSet's weak pointers since this supersedes them.

Add asGeneric() methods to generic readers, builders, and interfaces which casts them to the same type but with different generic parameters.