Module sync

Synchronization mechanisms based on the Python GIL.

With the acceptance of PEP 703 (aka a “freethreaded Python”) for Python 3.13, these are likely to undergo significant developments in the future.

Structs

• GILOnceCell
  A write-once cell similar to once_cell::OnceCell.
• GILProtected
  Value with concurrent access protected by the GIL.