Implemented REPL history search for text before cursor.
Stepping through and searching REPL history share the same implementation.
Added command, handler, keys for REPL history search.
Changes default keybindings for repl history search.

Default keybings are Ctrl+Alt+Up and Ctrl+Alt+Down.
Retained input handling + empty prefix search.

- Retained input is always last history entry in forward history search.
- History search with empty prefix (nil or "") steps through history entries.
Input modificaton resets history search.
Changes for pull request comments.

* Only one key binding definition instead of 3 redundant ones.
* Implements suggestions for several code changes.