Get rid of SplFixedArray

[dktapps]

Problem description

SplFixedArray is crap and probably slower than regular arrays in 2024.

Regular arrays with linear indexes are heavily optimized in modern versions of PHP (think packed arrays), so the reasons to use SplFixedArray today are far less than they used to be. In addition, they also suck for static analysis on account of probably having offsets containing null due to lack of any way to pre-fill the array.

It's also an extra object to lurk in the GC root buffer for no good reason, since every Chunk uses it, as well as Inventory and various other things.

We'd also be able to get rid of this: https://github.com/pmmp/PocketMine-MP/blob/f7687af337d001ddbcc47b8e773f014a33faa662/tests/phpstan/configs/spl-fixed-array-sucks.neon

and probably a bunch of other errors buried in actual-problems.neon

Proposed solution

TL;DR: It sucks and we should scrap it.

We can do this without BC breaks in places where SplFixedArray wasn't exposed on the API.

However, some places do expose it and would require breaks to get the job done, so this is labelled as a BC break.

Alternative solutions that don't require API changes