Assert a function output is equal to another.
Pseudocode: a_function(a) == b_function(b)
use assertables::*; let a: i8 = -1; let b: i8 = 1; assert_fn_eq!(i8::abs, a, i8::abs, b);
assert_fn_eq
assert_fn_eq_as_result
debug_assert_fn_eq