Allow Chunk, HeightMap iterators to work when const

[dxrcy]

Creates const versions of iterators for both Chunk and HeightMap.
Adds const overloads for begin and end methods for both structures.

Example usage:
The chunk variable in the following code snippet can now be marked const, without the for loop complaining.

    const Chunk chunk = mc.getBlocks(start, end);
    for (BlockType block : chunk) {
        //
    }

This should not be a breaking change; existing methods and iterator types remain the same.

[rozukke]

Should be ready to pull in after this functionality has test coverage.

[dxrcy]

Should be ready to pull in after this functionality has test coverage.

How foolish of me to forget.

[rozukke]

Thank you!