Currently the macro always provides `CFn`, `RustFn`, `RustArgs`, etc.
Change this so that:
1. This information must be explicily requested in the invocation.
2. There is a new `FTy` field available that emits a single float type,
rather than a tuple or signature.
Additionally, add two new macros that create enums representing function
names.
Introduce `libm_test::for_each_function`. which macro takes a callback
macro and invokes it once per function signature. This provides an
easier way of registering various tests and benchmarks without
duplicating the function names and signatures each time.
