No std ThreadId

[Person-93]

Re-implement ThreadId using PyThread_get_thread_ident. This makes is usable in no_std context.

[clin1234]

lgtm

[davidhewitt]

Thanks, one comment on the FFI definition (see below).

Do you expect that the test suite (which does a lot of thread spawning) will need to change?

[davidhewitt]

There is also PyThread_get_thread_native_id. Do you know how these might differ?

I see that PyThread_get_thread_native_id is only available on some platforms, it might be challenging to define that availability with cfgs. (We could probably get it from the interpreter sysconfig when available, and make assumptions for "default" stable ABI builds.)

Also, we should use c_ulong as the return type here, to match the header.

The niche is interesting but we haven't made those part of the FFI definitions previously (e.g. many pointers could realistically be NonNull<T> rather than *mut T).

Suggested change

	pub fn PyThread_get_thread_ident() -> NonZero<u64>;
	pub fn PyThread_get_thread_ident() -> c_ulong;