* Fixed a bunch of incorrect arginfo and a few incorrect error messages.
* Passes mem check test with no leaks!
* WIP.
* Fix build warning that was causing Bazel build to fail.
* Added compatibility code for PHP <8.0.
* Added test_valgrind target and made tests Valgrind-clean.
* Updated Valgrind test to fail if memory leaks are detected.
* Removed intermediate shell script so commands are easier to cut, paste, and modify.
* Passing all Valgrind tests!
* Hoist addref into ObjCache_Get().
* Removed special case of map descriptors by keying object map on upb_msgdef.
* Removed all remaining RETURN_ZVAL() macros.
* Removed all explicit reference add/del operations.
* Added REFCOUNTING.md to Makefile.am.
