Commits on Aug 12, 2021

1. Cheaper and safe caching for py_capsule! and py_capsule_fn! …

   The `py_capsule!` and `py_capsule_fn!` macros generate `retrieve` functions
   that cache their results for subsequent calls.
   
   Prior to this commit, caching is done with a generated unsafe `static mut`
   item whose access is made thread-safe by a `std::sync::Once` on the side.
   However this synchronization is unnecessary since `retreive` takes a
   `Python<'_>` parameter that indicates that the GIL is held.
   
   This changes the cache to use safe `static` item instead, with `GILProtected`
   for synchronization-free thread-safety and `OnceCell` for interior mutability.
   As a bonus, this removes the need for cache-related `unsafe` code in
   generated `retrieve` functions.
   
   This adds a dependency to the `once_cell` crate, which can be removed when
   `OnceCell` becomes stable in the standard library:
   rust-lang/rust#74465
   
   Alternatively `OnceCell` could be replaced with `UnsafeCell` plus some
   `unsafe` code, effectively inlining the core of the logic of `OnceCell`.
   
   Fixes dgrunwald#263

   

   SimonSapin committed Aug 12, 2021
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