Include pure variant of decompress

[ryanreich]

A function
decompressEither :: ByteString -> Either String ByteString,
that never throws, should be included as a courtesy. Although it can be thrown together from the stream interface in Codec.Compression.Zlib.Internal, this is both not obvious and not trivial.

[ocramz]

@ryanreich see #9

[ryanreich]

Following your suggestion, I think this counts as a duplicate.