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.

Full support of age encryption

mktar-batch now supports non-interactive age calls, though I've added a
dependency on expect.

tarcat, tarls, and untar now know what to do with .age files. They also
now know what to do with Zstandard compressed files that use .zstd
(instead of .zst).

Refactored code into multiple 'packages'. One day these may be
independently installable. For now it's just an organization method.

Rewrote main README and added per-package READMEs.

Also: bug fix on whiched and whichvi. They formerly propogated an error
if $EDITOR or $VISUAL were set to a non-existant executable. Also
improved the inline documentation.