Fixing memory leaks and deprecations

[redbullmarky]

This one has fixes for a few outstanding items, generally relating to memory leaks or deprecations/compatibility.

• PHP8.5 removes zend_exception_get_default(). Code updated to use zend_ce_exception(), available since PHP7.0. Fixes php 8.5 compile error - zend_exception_get_default not declared #544
• Resolved memory leak on repeated setExceptionFilter / setModuleLoader / setModuleNormaliser calls. First bit needed to fix Memory issues with setExceptionFilter #543
• Resolved a memory leak with closures/circular refs. This one fixes Memory leak with closures #480 and Memory issues with setExceptionFilter #543
• Fixed persistent string crash on FPM/Apache graceful restart. Fixes Avoid uses of zend_string_dup/zend_string_copy when it will outlive the http request #432
• Also fixed a V8 deprecation - migrated Holder() → This(). No specific issue open afaik, but this one was stopping us building.

All tests pass (as do our own, and we have a fair few, albeit coving specific weird edge cases and things specific to the way we do things). Hopefully this is ok, @stesie ? Hoping to look at a few more too and (finally) perhaps look at the 3.0.0 thing. We've been building against V8 AND libnode, both working well, so perhaps I can get some of the docs polished in the next batch.