Make SLIME more MELPA friendly
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
C-c C-k in xref works again.
* slime.el (slime-all-xrefs): Know when to stop iterating over
buffer lines.
Remove slime-next-line/not-add-newlines
Regression test for slime-recompile-all-xrefs
Naive attempt at making the previous test more robust
Really fix the slime-recompile-all-xrefs test

(and make it depend on who-calls which is more commonly
 supported than who-macroexpands)
Try and avoid a stale slime-path