deflate_compress: use lazy2 compressor for levels 8-9
deflate_compress: use lazy2 compressor for levels 8-9

Instead of switching directly from the lazy compressor at level 7 to the
near-optimal compressor at level 8, use the lazy2 compressor at levels
8-9 and don't switch to near-optimal until level 10.

This avoids poor compression ratio and bad performance (both
significantly worse than level 7, and significantly worse than zlib) at
levels 8-9 on data where the near-optimal compressor doesn't do well
until the parameters are cranked up.

On data where the near-optimal compressor *does* do well, this change
worsens the compression ratio of levels 8-9, but also speeds them up a
lot, thus positioning them similarly vs. zlib as the lower levels (i.e.
much faster and slightly stronger, rather than slightly faster and much
stronger).  The difference between levels 9 and 10 is increased, but
that's perhaps the least bad place to have a discontinuity.

Resolves https://github.com/ebiggers/libdeflate/issues/85

ebiggers/libdeflate,ebiggers/libdeflate,ebiggers/libdeflate