This will enable us to `include!` the file to access these types in
`libm-test`, rather than somehow reproducing the types as part of the
macro. Ideally `libm-test` would just `use` the types from `libm-macros`
but proc macro crates cannot currently export anything else.
This also adjusts naming to closer match the scheme described in
`libm_test::op`.
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.
