Very incompressible data could sometimes produce no backreferences
which would result in no distance huffman code being created (as it
was not needed), so VERIFY the code exists only if it is actually
needed for writing the stream.
This commit adds a fully functional DEFLATE compression
implementation that can be used to implement compression
for higher level formats like gzip, zlib or zip.
A large part of this commit is based on Hans Wennborg's
great article about the DEFLATE and zip specifications:
https://www.hanshq.net/zip.html
