Commits on Aug 16, 2020

1. sync: Implement map methods of parking_lot fame …

   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.

   

   udoprog committed Aug 16, 2020
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