Get all of src/pfl under 80 columns

Remove Globals.priming.

Also remove references to it in code that isn't being tested in the
regression checking build-sequence, and so is probably bit-rotted.

Provide numvariant version of variant, which behaves like

with_flag(Globals.priming,SOME "") (variant avoids)

Document change/incompatibility in release notes.

trailing newlines in *.{sml,sig} files from src/ removed

Trailing newlines from SML files in src/ were rendered in HTML documentation.

Minor random fiddly changes

Delete trailing whitespace throughout src/

Populating initial pfl library.