Combinatorially monic left hand sides of rules of VarACSets

It may or may not be too strict that we require the left map $l: L \leftarrowtail K$ to be monic. This constraint is demanded for pushout complements to be well-defined.

@ndo885 points out that it may be ok if this map binds an attribute variable but is otherwise monic. There is already a function in `src/incremental/Algorithms.jl` that checks this in the incremental code `is_combinatorially_monic`. It would be easy to replace the `is_monic` check in `Rule` constructor of `src/rewrite/Utils.jl` with this function. However, this change should also be accompanying tests to verify that this is a safe relaxation of the constraint.