Float
Change `EXPONENT_` to `EXP_` and `SIGNIFICAND_` to `SIG_`. These are pretty unambiguous, and just makes for less to type once these get used.
Add a way to call MPFR versions of functions in a predictable way, using the `MpOp` trait. Everything new here is guarded by the feature `test-multiprecision` since MPFR cannot easily build on Windows or any cross compiled targets.