Refactored archive logic

Almost all (un)archive code now lives in bash libraries. To add support
for a new compression or encryption algorithm, updates only need to be
made in `archive.bash`, `unarchive.bash`, and the parser in
`mktar-batch`.

`mktar-batch` has eclipsed `mktar` when it comes to explicit command
line options. Much of the logic wqas removed. `mktar` now soleyl exists
as a clever utility that guesses encryption/compression preferences
based on the output filename. Filename is now mandatory.

Refactored tests to reflect these changes.

Standardized use of here-strings in a couple of places.