Refactor: turn the base32 wrapper into a static library

This library is meant for private use by Keysmith, to simplify linking to the base32 functionality in CMake files.

I've been working on a possible solution for issue #2 (closed) and this changeset dropped out as a side-effect of needing to simplify CMakeLists.txt files. Essentially this static library makes it easier to consume the base32 (wrapper) implementation in unit tests. In particular, CMake takes care of handling all the relative paths with library dependencies whereas for source dependencies we need to maintain these ourselves.