Pack K Bits
Converts a stream of bytes with 1 bit in the LSB to a byte with K relevant bits.
More specifically, it packs K unpacked bits (one bit per byte) into a single packed byte containing K bits and 8 - K zeros.
This block takes in K bytes at a time, and uses the least significant bit to form a new byte.
Example: K = 4 in = [0,1,0,1, 0x81,0x00,0x00,0x00] out = [0x05, 0x08]