Add hex int syntax - #1403

I was wondering if we could have a new Word primitive type that would:

1. use 0wDEADBEEFCAFEBABE literals (saw that SML had those, I'm willing to bikeshed the syntax),
2. be 64-bit unsigned on all platforms (this would likely include some JS shim, or "polyfill" as they call it -- either wrapping a pair of numbers or using bigint),
3. supported only casts into/from existing numeric types, and
4. all bitwise operations would need to be done on Words or Bytes, that is no more bitshifts on Ints.

This would be useful for both FFI and for more bit-twiddling, bitset-ey structures, etc. WDYT?