Rollup merge of #151900 - tgross35:num-internal-imp, r=Mark-Simulacrum

num: Separate public API from internal implementations

Currently we have a single `core::num` module that contains both thin wrapper API and higher-complexity numeric routines. Restructure this by moving implementation details to a new `imp` module.

This results in a more clean separation of what is actually user-facing compared to items that have a stability attribute because they are public for testing.

The first commit does the actual change then the second moves a portion back.