Drop liboath for base32 decoding

Take the first fully backwards compatible step towards a liboath free Keysmith. This change provides a "simple" drop-in replacement for the base32 decoding functionality of liboath, with a view towards supporting decoding into "secure memory" pointers (see issue #6 (closed)).

Getting rid of liboath toolkit will make Keysmith easier to build for various systems such as Android. See also issue #9 (closed)

Note: this MR builds on top of !25 (merged)