Closes #26: Properly order entries when merging and saving files.

For every REPL restart, two "merge-and-save" operations were being
performed. Used a `sly-mrepl--dirty-history' flag to prevent
that. Also notived that setting `comint-input-ring-size' like it was
being done does not actually increase the size of persistent
history. So we leave it at the global default used by comint, which
the user should be able to set, too.

* sly-mrepl.el (sly-mrepl--dirty-history): New variable.
(sly-mrepl-mode): Don't try to set `comint-input-ring-size'.
(sly-mrepl--send-input-sexp): Mark history dirty.
(sly-mrepl--merge-and-save-history): Silly off-by-one isn't fixed
like this.
(sly-mrepl--merge-and-save-history): Mark history clean.
(sly-mrepl--save-all-histories): Check `sly-mrepl--dirty-history'.