Remove incorrect comment from convert-hex-to-b64

First attempt at solution took code from an SO answer, but our final
solution was not a copy of this answer - hence the comment removal.
Simplify partitioning in convert-hex-to-64

Padding our hex string with 0s was incorrect - just assert that the hex
string has an even number of characters.
Extract functionality for hex string to byte seq

Our next challenge also takes a hex string and needs to work on bytes,
so extract this functionality to its own function for reuse.
Add test for challenge 2

Also add "-challenge" suffix on test names to avoid collisions with
functions under test.
Add solution to challenge 2

Add function to bitwise XOR the sequences of bits represented by the 2
given strings of hex. Output is a string of hex.