rustdoc
Rather than collecting a list of file names in `libm-test/build.rs`, just use a script to parse rustdoc's JSON output.
MathOp
for_each_fn
Create a new test that checks `for_each_fn` against `ALL_FUNCTIONS`, i.e. the manually entered function list against the automatically collected list. If any are missing (e.g. new symbol added), then this will produce an error.