Support for 128-bit ints

[bradcray]

As time goes on, interest in int(128) seems to be growing. This issue is a placeholder requesting to add such support for 128 bit integers, at least on platforms and back-end compilers for which it's reasonably supported.

My assumption is that the work here would involve the following major components:

• extending the compiler to support 128-bit ints in addition to 8, 16, 32, and 64
• adding overloads to common 128-bit math routines
• ensuring that resolution and related compiler logic handled this new width appropriately
• plumbing the type to LLVM and C back-ends as appropriate

A lot of this seems pretty reasonable and mechanical as long as the LLVM/C support is there. The part I fear the most is the degree to which we've (intentionally or un-) baked in assumptions about the default int width also being the max.