Function reverse_bits_len 

pub fn reverse_bits_len(x: usize, bit_len: usize) -> usize

Reverse the bits of an integer within a specified bit length.

Takes an integer x and reverses its bits within the least significant bit_len bits. For example, reverse_bits_len(0b101, 3) = 0b101 (reversed) = 0b101. reverse_bits_len(0b001, 3) = 0b100.