ENH: sparse: rewrite some lil methods in Cython for speedups
ENH: sparse: implement cythonized fancy setitem
ENH: sparse: use memoryview in _csparsetools
ENH: sparse: add scalar indexing fast paths to LIL
MAINT: sparse: document the _csparsetools.pyx file appropriately
MAINT: add _csparsetools to .gitignore
MAINT: drop Cython from .gitattributes (no longer in repo)
ENH: sparse/lil: minor improvements in LIL fancy indexing

As per review comments:

- explain and rename prepare_index_arrays
- indicate out-of-bounds column indices
- faster insertion in lil_fancy_get
- explain reason for preferring isinstance in scalar fast paths
Merge pull request #3356 from pv/lil-speed-cy

ENH: sparse: speed up LIL indexing + assignment via Cython