Use elided lifetimes instead of `'gc` for the short form of `Rootable!`

[moulins]

This allows using the short form even when a 'gc lifetime is already in scope.

For ease of future maintenance, the lifetime substitution is done by a procedural macro instead of a recursive tt-muncher.

[kyren]

LGTM! I am soooo tired of typing the full Rootable! incantation, and this makes it a lot clearer to me and less visually noisy too.