FUEL: remove fuel-help-short and supporting code

The feature is not used anywhere (not bound to any key) and the normal
fuel-help display works better than fuel-help-short anyway
FUEL: Use font-locks data to figure out if the token point is on is a vocab

If point is on "myvocab" in IN: myvocab fuel-help will now understand
that it is on a vocabulary token.
FUEL: Improved handling of documentation markup errors

The fuel-eval--retort-result-safe function can be used to get the
retort result and throw an error if the retort error is
set. E.g. (fuel-help "prettyprint") will now throw an error because
the markup is invalid instead of showing "No help available.."
FUEL: fallback to the all-words algorithm when search doesnt work

It's to hard to figure out the correct "in" and "usings" values for
fuel-eval-in-context when *fuel help* is looking at articles. Instead
of trying, fallback on the less precise word lookup algorith and
accept that it sometimes will lead you wrong.
FUEL: improve highlighting of LIBRARY:, HELP: and deprecated
FUEL: factor-find-in returns correct vocab when point is on the IN:-line
fuel.pprint: . needs to be escpaed to load prettyprint's help page
FUEL: some more chars that often are used in word names