"scoped_thread_local!(...)" can be improved

Currently it's impossible to store structs with lifetime bounds in a scoped thread local, largely because rust doesn't support HKT. However, with macros and associated types, the type system is just powerful enough to express the required semantics.

Using the example implementation below, one can write code like the following:

scoped_thread_local!(static EXAMPLE: for<'scope> &'scope MutexGuard<'scope, u32>);

The presence of 'scope allows a much wider range of types to be stored safely in a scoped thread local. Without this change the only way to get the same behaviour is via the use of unsafe code.

Example implementation:
https://github.com/Diggsey/rust/tree/scoped_thread_local_hkt

(While it requires no changes to the compiler, changes to the compiler could make it significantly less hacky...)

Start with the scoped_thread_local!(...) macro entry point and review the linked scoped_thread_local_hkt example implementation. Check how its associated types and 'scope lifetime express storage of lifetime-bound structs, then assess whether the approach can be adopted without compiler changes. Done means scoped thread locals safely support the illustrated MutexGuard type.