Allow FnMut in blocks?

[madsmtm]

Upstream issue: SSheldon/rust-block#12.

Should we allow Block<dyn FnMut()>, and how would we do it safely?

At the very least, we'd have to prevent any form of Block::copy/RcBlock::clone on these, as that could lead to multiple mutable references being present. In particular, it is difficult to prevent re-entrancy if the block is shared.

This decision is blocked on a resolution in #563, though we might still decide to allow FnMut in block2, even if we don't allow it in objc2, as most (non-@escaping, non-@Sendable) blocks (probably) really are safely mutable.

See also #168 (comment) for discussion on why FnOnce is impossible, we should document that somewhere more visible when tackling this issue.