sync: Implement map methods of parking_lot fame #2771
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hawkw
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Generally, this mimics the way `MappedRwLock*Guard`s are implemented in `parking_lot`. By storing a raw pointer in the guards themselves referencing the mapped data and maintaining type invariants through `PhantomData`. I didn't try to think too much about this, so if someone has objections I'd love to hear them. I've also dropped the internal use of `ReleasingPermit`, since it made the guards unecessarily large. The number of permits that need to be released are already known by the guards themselves, and is instead governed directly in the relevant `Drop` impls. This has the benefit of making the guards as small as possible, for the non-mapped variants this means a single reference is enough. `fmt::Debug` impls have been adjusted to behave exactly like the delegating impls in `parking_lot`. `fmt::Display` impls have been added for all guard types which behave the same. This does change the format of debug impls, for which I'm not sure if we provide any guarantees.
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This is a backport to
0.2.xof #2445 because that's where I currently need it.