Readd Snappy #285

Blosc dropped Snappy in a recent upgrade ( #283 ). So we had to drop it as well for now. However it would be good to readd Snappy support. Instead of linking to the C/C++ code in Blosc, it may be better to use a separate dependency like python-snappy. This would also help us to slowly move from compiling as much code in Numcodecs itself ( like using Blosc wheels #262 )