Assert a function output is less than or equal to another.
Pseudocode: a_function(a) ≤ b_function(b)
use assertables::*; let a: i8 = 1; let b: i8 = -2; assert_fn_le!(i8::abs, a, i8::abs, b);
assert_fn_le
assert_fn_le_as_result
debug_assert_fn_le