op_serializer fails on gates parameterized by algebraic expressions

[trevormccrt]

For example, if i create a circuit including cirq.X ** a/2 where a is a sympy symbol, op_serializer.py fails in the _check_value method with the following error:

ValueError: Expected type convertible to float but was <class 'sympy.core.mul.Mul'>

@Strilanc do you know if not supporting this is intentional, or has it just never been added?

[trevormccrt]

@dabacon any thoughts on this? I figure it might have something to do with what you guys want to support on the engine side of things.

[dabacon]

If your circuits have formulas you need to flatten them. See https://github.com/quantumlib/Cirq/blob/master/cirq/study/flatten_expressions.py.

We can consider making this automatic by default, but we need to be careful with too much automagic that people don't understand.

[peterse]

related: #2358

[dabacon]

This now uses the func arg language construction for a small subset of symbolic expressions.