Add Fixed width user defined integer literals

A number of our internal api's, such as safenum, require fixed width integer types. I'd like to consider what adding user defined fixed width integer types might look like.

The idea being that one could:

42i64 == static_cast<int64_t>(42LL)

1234u32 == static_cast<uint32_t>(1234UL)

etc.

Mostly a quality of life issue

This is probably blocked on the upgrade to msvc 2015, as vc12 lacks support for user defined literals