Fixed-size integers larger than Int64

Provide a family of `[U]Int128`, `256`, ... up to a reasonable threshold where flexibly-sized bignum becomes competitive (most likely 1024 bits). This should be done as generically as possible so that support for additional sizes can be added as needed with minimal complexity.

These types should conform to `FixedWidthInteger`.

No files, tests, or entry points are named. Start by reviewing the existing numeric type design and the requirements of Swift's FixedWidthInteger protocol, then define the supported widths and generic approach. Done means the requested fixed-size integer family is implemented with the required protocol conformance and a clear upper-bound decision.

Written by the indexing model from the issue text.