Auto merge of #55704 - Nemo157:pinned-generators, r=Zoxc

Auto merge of #55704 - Nemo157:pinned-generators, r=Zoxc

Use pinning for generators to make trait safe

I'm unsure whether there needs to be any changes to the actual generator transform. Tests are passing so the fact that `Pin<&mut T>` is fundamentally the same as `&mut T` seems to allow it to still work, but maybe there's something subtle here that could go wrong.

This is specified in [RFC 2349 § Immovable generators](https://github.com/rust-lang/rfcs/blob/master/text/2349-pin.md#immovable-generators) (although, since that RFC it has become safe to create an immovable generator, and instead it's unsafe to resume any generator; with these changes both are now safe and instead the unsafety is moved to creating a `Pin<&mut [static generator]>` which there are safe APIs for).

CC #43122