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.