Make SimSYCL thread-safe

[fknorr]

Overall, SimSYCL has very little shared mutable state:

Fiber scheduling always happens inside a parallel_for, so it will never cross thread boundaries. The corresponding cooperative_schedule and "return fiber" can be made thread-local.

For where there is globally visible shared state (system config, USM allocations, buffer access validation), I've introduced a global lock (named system_lock, which is backed by a singleton mutex). All shared mutable state is now explicitly wrapped in a mutable shared_state<>, which only grants access to its interior when a system_lock is in scope.

To guarantee command-group ordering across threads without tracking dependencies explicitly, submit (and all similar functions) hold the system_lock while they're running.

With this patch, Celerity works fine on my machine ™️ with workarounds removed.

[PeterTh]

LGTM