Comprehension nesting limit validator

[TristonianJones]

Introduce a comprehension nesting limit validator which checks to see
if more than limit non-trivial comprehensions are nested within each
other.

A trivial comprehension is one in which the iterRange is an empty list,
such as occurs during cel.bind() calls. The computational overhead
of these kinds of comprehension ASTs is nominal and should be covered
by cost estimation toolilng.

Closes #420