Support lz4 block format

[milesgranger]

To help with dask/fastparquet#580

[martindurant]

benchmarks look good

[milesgranger]

This will add block format for lz4, but not (yet) _into options as lz4 doesn't support de/compression into slices, only outputting new Vecs.

There is a possibility _intos could be added later as lz4_flex may be adding this API in the future, trade off there being no guarantee their the API would follow a similar Python API as lz4 crate does now; so would almost certainly require re-implementing the store_size component. Could always get creative and use some unsafe code to trick whatever slice we have into a Vec if it's really desired; as I'm pretty sure lz4 crate won't be changing its API anytime soon; but have opened an issue anyway

[milesgranger]

@martindurant is this good with you? I'll come back for the _intos depending on what becomes of that issue I made.

[martindurant]

Yes, totally good.
It turns out there is also a hadoop-specific version of lz4 that was used by map-reduce, but I think we can safely ignore it.