Better integrate the 0,1,2,4 bytes mode

[lemire]

Following this PR #26 we now have code that can use a 0,1,2,4 byte encoding. However, it is basically achieved through pure code duplication. Worse: it does not benefit from @aqrit 's latest improvements.

Obviously, we could do better.

[aqrit]

The latest improvements transcribed from streamvbyte_1234_encode_SSSE3
https://gist.github.com/aqrit/55167c6638e925ddca4b10ee8fba4735
33% faster, 75% smaller.

If I ever find a good way to organize the files and tables... I'll turn this into a commit.