codec, baseconv: Cleanup of the various binary encoding functions.

codec, baseconv: Cleanup of the various binary encoding functions.

The new codec interface provides an object-based interface to binary
encodings.

The baseconv code is a new unified implementation of Base64, Base32 and
hex encoding and decoding, built from a digit-at-a-time base conversion
core.  It implements a number of variations on the basic encodings,
including the URL-and-filename-safe variant.

I've adapted the current tests to use the new test program, but that's
not long for this world.