igzip plugin

[nemequ]

Intel released an open-source (MIT) gzip implementation with faster compression but lower ratio than zlib. Some details are at https://software.intel.com/en-us/articles/igzip-a-high-performance-deflate-compressor-with-optimizations-for-genomic-data and the code is at https://software.intel.com/sites/default/files/managed/2d/63/igzip_042.zip

Unfortunately it will take a bit of work to get ready for Squash—for one thing the choice between gzip and deflate, as well as the compression level (there are two) is a compile-time thing. Also, it doesn't include a decompressor, but that is a problem I need to figure out for zopfli anyways :(