Adding eq_symmetric rule

This adds a new rule used to elaborate implicity reasoning with equality symmetry. This addresses issue #43 (closed).

merged

mentioned in commit 5ad781d4

Merging this. Rules should also added to the classification tables at the beginning of the rule list. In this case the tautology table and dedicated table for Carcara rules. I will do this in an extra commit.

mentioned in issue #43 (closed)