swift234

[stdlib/prototype] Replace Integer Prototype

This drops BigInt support (for now) and replaces the low level integers
with signed and unsigned variants, much like what we have currently.
However:

* The protocols make sense, unlike today
* Generic conversion between integer types is implemented
* Heterogeneous comparison is implemented
* Masking bit-shift operators are implemented
* Non-masking shifts handle negative shifts and overshifts
* Heterogeneous bit shifting is implemented

The Protocols and Word-sized fixed-width integer types defined here
should still make a fine basis for BigInt, which can be implemented
next-ish.

Because of missing intrinsics, 32-bit platforms can't support Int128
yet, but we can make it work by applying the same multi-precision
arithmetic algorithms we'll develop for BigInt.