Create blocks with BLOCK_HAS_SIGNATURE

[goffrie]

https://github.com/madsmtm/objc2/blob/dd6c804d1e189fd8c9b313d8c1901ee9e7286551/crates/block2/src/concrete_block.rs#L189

Some newer Apple APIs appear to fail in unexpected ways when called with blocks that don't have signatures. It would be useful to at least have some way of manually specifying a signature, though unsafe. Generating it automatically would be even better of course, though it's an interesting question how to actually do so.