Initial SLIME REPL integration support.

Added support for SLIME REPL integration for uv branch (see #65).
`(as-repl:start-async-repl)` starts the event thread and installs REPL hook so user input is eval'd in the context of the event loop, `(as-repl:stop-async-repl)` stops event thread and uninstalls the hook. If event loop dies for some reason (such as `(as:exit-event-loop)` call), the hook is uninstalled automatically.
Had to improve notifier support a little bit.