The compiler-synthesised trampoline path (previous commit) covers
every closure signature on demand; the hand-rolled stdlib impls
were only for two specific shapes (`Closure() -> void`,
`Closure(bool) -> void`) and are now strictly redundant.
Kept: the `Block` struct, `BlockDescriptor`, the
`_NSConcreteStackBlock` extern decl, and the shared
`__sx_block_descriptor` global. The compiler-emitted code
references all four; users still need to `#import
"modules/std/objc_block.sx";` to bring them into the module.
Removed: `__block_invoke_void`, `__block_invoke_bool`, and both
`impl Into(Block) for Closure(...) -> void` blocks. Replaced with
a comment block explaining how the compiler now handles the cast.
After this commit, `xx my_closure : Block` works for ANY closure
signature with no per-signature stdlib boilerplate. 189/189
example tests pass; chess on iOS-sim green.
556e4e12ea