Rather than implementing `Drop` for all three lock guard types to
restore the lock analysis' per-thread state, let lock guards own
values of a new type, `LockStateGuard`, with the appropriate `Drop`
implementation. This is cleaner and shorter, and helps us implement
`RwLock::downgrade` in a later commit.

Implement `RwLockWriteGuard::downgrade` for `lock::vanilla` and
`lock::ranked`, to downgrade a write lock to a read lock.

The lock analyzers in the `wgpu_core::lock` module can be a bit
simpler if they can assume that locks are acquired and released in a
stack-like order: that a guard is only dropped when it is the most
recently acquired lock guard still held. So:

- Change `Device::maintain` to take a `RwLockReadGuard` for the device's
  hal fence, rather than just a reference to it.

- Adjust the order in which guards are dropped in `Device::maintain`
  and `Queue::submit`.

Fixes gfx-rs#5610.