Integer utilities module

A new module providing additional functionality on the standard library integer types (and more generally, any fixed-width integer type), including:

- [x] - bitwise rotation
- [ ] - full add / sub (with carryin/carryout)
- [x] - saturating arithmetic (functions, not operators)
- [x] - division with rounding control
- [x] - shift with rounding control
- [x] - "true" mod function (Euclidean division)
- [x] - GCD
- [ ] - division by constant (precompute magic multiply and shift a la libdivide)
- [ ] - modulus by constant (use Lemire's method)

No source file or test is named. Start by reviewing the repository's existing module layout and integer-related tests, then determine the scope and API for one unchecked item in this checklist. Done means the selected integer utility is implemented for fixed-width integer types and covered by tests.

Written by the indexing model from the issue text.