Repl history fixes and improvements

Make repl history navigation smarter as documented in the commit logs.

Notably:

- M-r/M-s (or what slime-repl-previous/next-matching-input is bound to) can be used similar to C-s: when pressed multiple times then jumps to the next match with the same regexp
- minibuffer reading of regexp search has its own history, and defaults to the previous input
- pressing M-r while reading the regexp in the minibuffer initiates searching with the previous regexp (like isearch)
- only the sending of the repl input resets the index of navigation history
- drop the feature that the typed-in text at the repl is used as initial history search regexp (the new input method is much more flexible)
- while navigating, display in the minibuffer which repl history index we are at, and what regexp is used for filtering currently
- when reaching the most recent end of the history, then clear the repl input