Possible fresh variable name collisions

[cameel]

Currently Prolog.rewriteVariable() works with the assumption that hashing a variable name together with an integer won't ever produce a collision with an existing variable:

https://github.com/leonardoalt/SolPrologV2/blob/47bb8ec9253022ae91992eda144c0b4ca19914df/src/Prolog.sol#L77-L82

A collision unlikely but not impossible. We may want to use a safer method of obtaining fresh variable names.