Rebalance compression levels.

[klauspost]

This will rebalance the compression levels to have a more even distribution of compression levels. The general trend of this is to make compression faster, which means that a given compression level (except 1 and 9) will be faster.

The existing implementation favors compression, and from level 5 and up, very small compresssion improvements are added for very high computation cost.

Rough guidelines: Previous level 3 speed is now equal to level 6.

New default is level 5, which in most cases offers a quite good compression, without getting diminished returns from additional computation power.

See "rebalance" tabs for benchmarks of rebalanced levels:

https://docs.google.com/spreadsheets/d/1nuNE2nPfuINCZJRMt6wFWhKpToF95I47XjSsc-1rbPQ/edit?usp=sharing

[willglynn]

One thing I note about the original values: they seem to correspond to the compression levels in zlib. Even the compress/flate constants mirror the zlib constants. Someone who's used zlib before might assume that Go compression levels 0-9 are similar to zlib (I did!), and prior to this changeset, they'd be right.

I like the idea of having more speed/size tradeoffs available, but I'd also like a note in the docs saying that the compression levels work differently than zlib.

[klauspost]

@willglynn You are right. It makes good sense to document why these values are different than zlib.

That said, in my tests zlib has always performed ~1 level worse than go deflate in terms of compression. Noting that you shouldn't expect settings to carry over 1:1 makes sense.