Add const to lots of pointers

[MaxKellermann]

No description provided.

[Girgias]

The only part I am unsure about is adding the const pointers to the compare handlers as this is an API break for extensions, and at least needs to be documented in UPGRADING.INTERNALS.

Therefore could you spin it out into a seperate PR? Because if we start chaning the object handler API it probably makes sense to change int returns to zend_result or bool as well.

[MaxKellermann]

Therefore could you spin it out into a seperate PR?

Aye. I splitted the first commit and removed lots of hunks, leaving only the ones that are necessary for the following commits.

Because if we start chaning the object handler API it probably makes sense to change int returns to zend_result or bool as well.

That part of the API bothered me as well, but yes, maybe I'll make a new PR which refactors everything to use proper types and proper const-ness.