Safer thread parkers

[faern]

Continuing the work of trying to remove unsafe from methods/functions that actually can't result in UB.

I know the spin based generic thread parker is also in the other PR (#106). But it was kind of needed here to get rid of unsafe prepare_unpark on (almost) all platforms. I will remove that commit from the other PR if/when this is merged.

I left the two constructors of the Windows thread parkers as unsafe. The documentation for GetModuleHandleA suggests it has to be carefully called and used in a multi threaded environment. At the same time I'm not sure what exactly could cause it to fail, or if that is even possible in our setup. If you have more information on the topic, maybe we can safely mark them as safe?

[Amanieu]

The warnings about GetModuleHandle can be safely ignored I think. This only applies to DLLs that might be dynamically unloaded while we are still using them. This shouldn't apply to core system DLLs like ntdll.dll or api-ms-win-core-synch-l1-2-0.dll. However if you want to be absolutely sure then you can use LoadLibraryA instead, which is slower (but it should still be fine considering this is one-time initialization).