Support larger number of permits

[kornelski]

I'm planning to use permits to limit peak memory usage, and for that u16 is very limiting.

I don't know if a small type is critical for performance here, so just in case I've opted for u32 instead of usize.

Related to #2605 #2598

[Darksonn]

It looks fine to me, but I would like to hear from @hawkw.

[Matthias247]

Due to pub(crate) const MAX_PERMITS: usize = std::usize::MAX >> 3; that might not work on 32bit platforms, where you MAX_PERMITS would then be lower than u32::MAX. The counter which accumulates the permits as well as some other metadata in a single usize value could then underflow or overflow.

Maybe it could work if the u32 value is additionally checked against the maximum allowed value in some additional places - but I haven't checked that so far.

[kornelski]

I've added an assertion and rebased.

This isn't yet exposed in this PR (#2598), so there are no more checks to add yet.

[hawkw]

Looks good to me.

I believe the limit of a u16 was inherited from the previous Semaphore implementation (semaphore_ll) and shouldn't be necessary with the new implementation --- unless @carllerche, who wrote the old semaphore, can think of a reason it matters?