Completed arbitrary field processing.

Component "variable/string/pcre/field" now implements arbitrary field
processing via
for_string_line_fields_split_on_pcre_field_and_separator(). The
implementation is reasonably efficient in both space and time and
arguably more generalized (if less efficient) than the "awk" family of
traditional field processors. Such processors do *NOT* (commonly,
anyway) support such much-vaunted features as arbitrary lookahead and
lookbehind, whereas the above function does. (Hurrah! Hurrah! And such.)