Add a convenience API for forking the current hub

[szokeasaurusrex]

Add an API for forking the current hub as a convenience wrapper around new_from_top on the current hub, while avoiding Arc-cloning.

This supports the common pattern of creating a per-message scope/hub so information does not bleed between processed messages.

Possible naming mentioned in discussion included Hub::new_from_current or Hub::fork_current.

Relevant context: the existing Hub::with pattern is deprecated in favor of Hub::current(), but Hub::current() returns Arc<Hub> while new_from_top takes AsRef<Hub>.

[linear-code]

RUST-257