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.

Big archival update

mktar now experimentally supports SHA256 checksums and age encryption.

mktar-batch is a version of mktar that strips out the 'clever' algorithm
detection and includes a batch processing/common passphrase mechanism.

Updated tests for mktar.

Quick fix to error text

Fix mktar

SHA1 checksum files were created regardless of existence of corresponding
file. Worse yet, they weren't actual checksums.

Formatting fixes; Help message fixes

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.

Specify full path to POSIX utilities; Clean up scripts' output

Added tarls; Update to mktar for archiving directories

Initial commit of cleaned-up dev branch