Fix byte-compile warnings in contrib/slime-fontifying-fu.el.

When loading a Slime that has already been byte compiled, the explicit
byte compilation code in contrib/slime-fontifying-fu.el generates
warnings.  Use the same technique as used in slime-parse.el -- only byte
compile if it's necessary.
Fix source locations when recompiling from an xref buffer.

* slime.el (slime-recompile-locations): pass full filenames to
swank:compile-multiple-strings-for-emacs