Pass entire int array to picosat_add_lits once per clause, in AddClauses()

This improves performance when passing a bunch of clauses, by reducing the number of times we need to call into C. Instead of once per literal, it is once per clause. 
For consistency with other calls into C in the library, please add a comment above the `C.picosat_add_lits` call with the signature of the C function being called, like so:
```go
// int picosat_add_lits (PicoSAT *, int * lits);
C.picosat_add_lits(p.p, (*C.int)(&clause[0]))
```