Add AFL test case

I changed it to use TestAllTypes instead of a trivial struct, which will probably get more coverage quicker.

I'm not really sure how Cap'n Proto has worked at all on GCC 5. For some reason this only ended up being tickled after e6e29122.

Fixes #442.

Fixes https://github.com/sandstorm-io/ekam/issues/13
Fixes https://github.com/sandstorm-io/ekam/issues/11

Fix build with -fno-exceptions

This includes making builders a little more tolerant of corrupt data. Note that our threat model generally does not expect this tolerance -- we expect that builders always contain either structures created locally or copied in from a reader, which does a certain amount of validation in itself.

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.