[3.12] gh-99108: Release the GIL around hashlib built-in computation (GH-104675) (#104776)

gh-99108: Release the GIL around hashlib built-in computation (GH-104675)

This matches the GIL releasing behavior of our existing `_hashopenssl`
module, extending it to the HACL* built-ins.

Includes adding comments to better describe the ENTER/LEAVE macros
purpose and explain the lock strategy in both existing and new code.
(cherry picked from commit 2e5d8a90aa633ff0bebc9b2b8e21eea389937b19)

Co-authored-by: Gregory P. Smith [Google] <greg@krypto.org>