Changed default level 18 (large input)

Changed default level 18 (large input)

Previous -18 : 4.7 MB/s, R:3.833
New -18 : 5.1 MB/s. R:3.825

It's a better fit within -17 (6.8 MB/s) and -19 (4.0 MB/s)
The new level 18 also uses significantly less memory.
And, it makes a good transition between level 17 (mml5)
and level 19 (mml3).
Up to now, there was no level with mml4.

(note : minmatch setting can have a large impact on some (specific) datasets)