Baseline implementation of division by constants

When dividing integers by compile-time constants, compilers (LLVM) can be trusted to convert those to a sequence of multiplication and shift.

That doesn't work so well when the constant isn't known until runtime.

This crate implements a simple strategy with reliable performance.

The basic PartialReciprocal should be compiled to a constant-time fast path, and can handle every divisor except 0, 1, and u64::MAX.

The slightly more complex Reciprocal can also divide by 1 and u64::MAX, at the expense of one branch in the fast path: we must treat divisions of u64::MAX specially.