Add fastest block-splitter variant

[Cyan4973]

this fastest variant is less precise but its speed is suitable for fast strategy. since it remains within noise level (<2%) even at negative compression levels (tested up to -7).

silesia.tar :

level	dev	this PR	savings
1	73,422,067	73,193,855	-228,212
2	69,503,582	69,309,934	-193,648

The impact for negative compression levels is negligible, likely due to the disabling of the huffman literal compression stage, but nonetheless remains generally positive by a small margin.