improve 2-watched and representation of literals
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...

regb/scabolic,RomanHargrave/scabolic