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

This PR gains 5...50x speedups in LIL matrix setitem/getitem by writing the fancy indexing code more carefully. I also add fast paths for scalar indexing (~5x faster).

The fancy getitem speed is now not far from CSR/CSC. Fancy setitem is also not far from CSR/CSC (and for changing sparsity pattern, LIL can now be much faster than CSR/CSC).

Benchmarks: https://gist.githubusercontent.com/pv/9102868/raw/a256bfe058b44131f8acba0f41ab90eafacbe127/gistfile1.txt