Fix bug with diff or unary ops that have same static representation

Previously, for testing whether two diff() expressions or two unary ops were
the lead/lag of each other, the preprocessor would test whether they have the
same static representation. This is ok for simple expressions (e.g.
diff(x(-1))), but not for more complex ones (e.g. diff(x-y) and diff(x(-1)-y)
should not be given the same auxiliary variable).

This commit fixes this by properly constructing the equivalence relationship
and choosing a representative within each equivalence class. See the comments
above lag_equivalence_table_t in ExprNode.hh for more details.

Closes #27