Add hooks for extending arglist lookup.

Useful for example in [slime proxy](https://github.com/3b/slime-proxy), which wants to show arglists for parenscript functions/macros in addition to or instead of those defined in the host lisp.
I'm confused about the protocol of `*arglist-hooks*`. It doesn't seem to match the `%arglist` interface which returns `:not-available` when the thing is not found.