commonly used functions to convert an arbitrary binary string into
a hexadecimal digit representation and back again.  These are often
(and often differently) implemented in Python.  Best to have one
common fast implementation.  Specifically,

binascii_hexlify(): a.k.a. b2a_hex() to return the hex representation
of binary data.

binascii_unhexlify(): a.k.a. a2b_hex() to do the inverse conversion
(hex digits to binary data).  The argument must have an even length,
and must contain only hex digits, otherwise a TypeError is raised.