Update test traits to support `f16` and `f128`, as applicable. Add the
new routines (`fabs` and `copysign` for `f16` and `f128`) to the list of
all operations.
Once we start addinf `f16` and `f128` routines, we will need to have
this cfg for almost all uses of `for_each_function`. Rather than needing
to specify this each time, always emit `#[cfg(f16_enabled)]` or
`#[cfg(f128_enabled)]` for each function that uses `f16` or `f128`,
respectively.
Now that we are using rustdoc output to locate public functions, the
test is indicating a few that were missed since they don't have their
own function. Update everything to now include the following routines:
* `erfc`
* `erfcf`
* `y0`
* `y0f`
* `y1`
* `y1f`
* `yn`
* `ynf`
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.
