eeb5b782a6
Also, `MTRef<'a, T>` is a typedef for a reference to a `T`, but in practice it's only used (and useful) in combination with `MTLock`, i.e. `MTRef<'a, MTLock<T>>`. So this commit changes it to be a typedef for a reference to an `MTLock<T>`, and renames it as `MTLockRef`. I think this clarifies things, because I found `MTRef` quite puzzling at first.