chunk: Introduce a constant for chunk height.
Use CHUNK_HEIGHT in place of 256.
And update locations using 128 as well.
utilities/coords: Add a couple more iterators.

xz-coords are presented as a constant because they weigh just over 1KiB on
CPython and are computationally cheaper than an invocation of product() on
both CPython and PyPy.

CPython before: 34.6us
CPython after: 15.2us
CPython speedup: 2.28x
PyPy before: 146us
PyPy after: 1.29us
PyPy speedup: 113.18x
And use new coord helpers.

Tests lost about 4 seconds of runtime on PyPy. Encouraging!