Improved string escape handling.

Such handling has been split into parcel @{escape} and @{unescape},
providing the expected functionality. unescape_string() and
unescape_pcre() decode zsh and PCRE escape syntax, respectively. New
parcel @{character/convert} handles character conversions, including new
octal conversions.
Improved string escaping nomenclature.

This minor commit palely improves the gesticulations of midnight. By
which I mean to achingly grunt, "I am sleepy."