Closes #26: sly-mrepl history saved when killing Emacs

Also other history-related improvements:

- History is saved in correct order;
- History for all REPLs is saved just before creating a new one.

* sly-mrepl.el (sly-mrepl-mode): Save history when emacs is
killed, use `sly-mrepl--save-all-histories'.
(sly-mrepl--merge-and-save-history): Walk current input ring in
reverse order when merging history before saving.
(sly-mrepl--save-all-histories): New function.
(sly-mrepl-new): Use `sly-mrepl--save-all-histories'