Bit shifting for power of 2 division

From some quick and rudimentary testing, a `>>` operation on several `boost::multiprecision` types is significantly more performant than the existing division operand, when the divisor is a power of 2. I'm expecting there to be a reason this check and shortcut isn't implemented, hence an issue and not a PR :)

No files, tests, or entry points are named. Start by locating the division implementation for the mentioned Boost.Multiprecision types, then compare it with their right-shift operations for power-of-two divisors. Done means establishing the applicable types and cases, preserving correct division behavior, and demonstrating the performance change with focused benchmarks or tests.

Written by the indexing model from the issue text.