Since this header is included by everyone, and units.h has lots of templates, this seems like it could significantly improve build times.

Don't allow kj::maxValue / kj::minValue for Guarded types, since they'll break down when CAPNP_DEBUG_TYPES is off.

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.

See: https://capnproto.org/news/2015-03-02-security-advisory-and-integer-overflow-protection.html

This commit as-is is the result of wading through two years of merge conflicts. It does not build as-is because new code added in that time hasn't been converted over.

The problem with kj::range() is that if the inputs are not recognized as the same type, the typechecker can get annoyed.

The existing error message suggests this case can only arise if there is a
a bug in the library. However, malformed input can trigger the error too.
In particular, the error gets thrown when a non-double far pointer resolves
to another far pointer.

Closes #408. Perhaps the same bug as in commit cff9ba05?