Move symbolic_counting_utils.py to a new module in qualtran/symbolics/

[tanujkhattar]

With improved typing, we want to use types like SymbolicInt etc everywhere, including qualtran/_infra. This leads to cyclic imports since qualtran/resource_counting/ depends on qualtran/_infra. Moving symbolic utils to its own module helps avoid these cyclic imports and provides the right place to add future utilities for dealing with symbolics