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`
Currently there is a combination of names starting with
`multiprecision_`, `mp_` and `multiprec_`. Update so `multiprecision_`
is always used when a long form makes sense, `mp_` otherwise
(eliminating `multiprec_`).
