Use SWANK:PACKAGE-STRING-FOR-PROMPT for sly-mrepl

* contrib/swank-mrepl.lisp (package-prompt): Removed.
(send-prompt): Use SWANK:PACKAGE-STRING-FOR-PROMPT

* lib/lisp/swank.lisp (:compile-toplevel): Export
PACKAGE-STRING-FOR-PROMPT
Consolidate `sly-display-warning' into `sly-warning'

* sly.el (sly-warning): Renamed from `sly-display-warning'

* contrib/sly-fontifying-fu.el (sly-search-suppressed-forms)
(sly-extend-region-for-font-lock): Use new `sly-warning'

* contrib/sly-mrepl.el (sly-mrepl--commit-buffer-name): Use new
`sly-warning'.
Fix source locations when recompiling from an xref buffer.

From https://github.com/slime/slime/pull/175

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

(cherry picked from SLIME commit
0249a079921b29ceb720c6524c30a7ee8d60a848)

Conflicts:
	ChangeLog
	slime.el
Improve support for strings in (:export ...) clause

From https://github.com/slime/slime/pull/145

* contrib/sly-package-fu.el (sly-export-symbols): Handle strings.
(sly-defpackage-exports): Use `sly-export-symbols' and strip
double quotes.
(sly-frob-defpackage-form): Don't skip package name in case
(:export ...) is immediately following.
(sly-determine-symbol-style): Support double quoted styles.
(sly-search-exports-in-defpackage): Handle strings.
(sly-remove-export): Avoid trailing whitespace.
(sly-find-possible-package-file): Use cl-dolist to catch cl-return.

(cherry picked from SLIME commit
c8f64ef1313df24db7cde8fbf6e35e24438e2d28)

Conflicts:
	contrib/ChangeLog
	contrib/sly-package-fu.el
Start a bare-bones NEWS.md file