soundness issue with Lock

[dherman]

The Lock API currently keeps the exposed internals alive longer than the lock is held (in particular, as long as the lifetime of the Scope the handle belongs to). This is unsound.

Since Handle is an auto-deref type, it should be possible to impl Lock for a borrowed reference to the target of the handle, instead of the handle itself, which should correctly bound the lifetime of the exposed internals.

But this also means that to trigger the auto-deref, we'll need to replace vm::lock with a method on Lock, so that it can be performed by a method call on the handle, which implicitly borrows the target.

[dherman]

Candidates for the method name: tap, uncover, unwrap, unpack, open, operate, pierce, puncture, probe, peel, seize, grab, take, zoom, focus

I like "grab". It's kind of evocative of temporarily taking hold of the value, and also hints at grabbing the lock.