@kloepper pointed out a while back that every compiler you've ever heard of supports this. Plus, it's more concise, it's not prone to copy-paste errors, and it looks nicer.

At the time I wanted to remain consistent and I didn't feel like spending the time to update all my existing code. But, every time I've added a new header since I've cursed the include guard, so I finally broke down and changed it.

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.