Data races in late-static

Affected versions of this crate implemented Sync for LateStatic with T: Send, so that it is possible to create a data race to a type T: Send + !Sync (e.g. Cell).

This can result in a memory corruption or other kinds of undefined behavior.

The flaw was corrected in commit 11f396c by replacing the T: Send bound to T: Sync bound in the Sync impl for LateStatic.