pushes back an array of bytes by copying it to the front of the pushback buffer. note that the
hash is created from the data read originally, i.e., if data different from the data read is
pushed back and read again, the hash does not change.
pushes back a byte by copying it to the front of the pushback buffer. note that the hash is
created from the data read originally, i.e., if data different from the data read is pushed
back and read again, the hash does not change.