Actually implement integer overflow guards

fix typo: unconstained -> unconstrained

add David Renshaw to CONTRIBUTORS

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.

omit needless memset

remove unneeded kj-async dependency in Makefile.am and capnp-json.pc.in

fix various bugs in canonicalization

fix typo in canonicalization docs

fix bug where PointerBuilder::getPointerType() can corrupt PointerBuilder::segment

update mention of non-existent 'stepBits' field