Macro libm_helper

macro_rules! libm_helper {
    ($t:ident, funcs: $funcs:tt) => { ... };
    ({$($func:tt);*}) => { ... };
    ((fn $func:ident($($arg:ident: $arg_typ:ty),*) -> ($($ret_typ:ty),*); => $libm_fn:ident)) => { ... };
}