FUEL: words-matching outputs an assoc so pick only the keys which are the words + test
FUEL: corrected name of *fuel xref* buffer
FUEL: big refactoring of fuel-xref-mode and the fuel.xref vocab

The big difference is that links to words are now grouped by vocab and
sorted alphabetically which I think makes *fuel xref* much more usable
when you have lots of words in the list
fuel.xref: refactor of normalize-loc and group-xrefs per mrjbq7s comments
math.vectors: new word v>integer = [ >integer ] map
math.vectors: changes so that some vocabs that can use math.vector words does it
project-euler: [ + ] 2map -> v+