Fix and simplify world::for_each_block_optimized

[verglasz]

First time contributor, so please let me know if I'm doing anything wrong.

I was looking at the source for no particular reason at all and decided to take a stab at this:

MCHPRS/crates/world/src/lib.rs

Line 84 in 76b9b8c

// TODO: I have no idea how to deduplicate this in a sane way

finding a bug in the process.

This PR replaces the duplicated implementation of for_each_block_optimized and
for_each_block_mut_optimized in the mchprs_world crate with a simpler implementation
that abstracts the iteration logic.

Also this adds tests for the iteration logic and for the for_each_block_optimized function
(which is how I realised that the current implementation had a bug that could lead
to some non-air block being skipped, see #168 for more details); let me know if
they're too verbose or if there should be other cases covered.

With the new implementation #168 should be fixed, since the new iteration special-cases
the bounding box boundaries and otherwise checks the block in batches corresponding
to chunk sections.

[Paul1365972]

Good catch, I initally wrote this code to only work for entire plots, but I seems like I somehow forgot that invariant when making it more generic, thank you very much!
The new code might perform worse as we call block_pos_in_chunk_section_between for each chunk, instead of handling the edge cases separately, but I would guess this is negligible.

[verglasz]

Yeah, I think it’d need benchmarking to see what’s the best performant approach for that, couldn’t think of anything particularly clever to do around it but it may be worth seeing if checking whether we’re in one of the chunk sections on the edge or maybe iterating over those separately from the interior do perform better.