Fix github issue #193.  Respect compilation-ask-about-save.

* slime.el (slime--save-some-buffers): New helper.
(slime-compile-file): Use it.

* slime.el (slime-echo-arglist): Renamed back from
slime-show-arglist as there are still uses.
Fix some compiler warnings.

* slime-tests.el (slime-tests--undefine-all): Use cl-remprop
instead of remprop.
(slime-test--compile-defun): Fix typo.
* swank/backend.lisp (codepoint-length): Deleted. No longer used.
Fixed anchor and cross-reference in slime.texi.

Makeinfo doesn't like colons in reference targets. (Closes pr #176.)