benchmark script adapted to last version

This uses the satlib option that warmup the JVM and
benchmarks the whole directory.
improve 2-watched and representation of literals

the 2 literal watched are implicitly the two first elements
of the array of literals in each clause. We keep moving literals
around to maintain this invariant.

Literals are now represented using positive integer, alternating
between pair and impair. 0 is v1, 1 is -v1, 2 is v2, 3 is -v2, and so
on...