Improve support for strings in (:export ...) clause

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