ParamResolver does not have evaluatable repr

[kevinsung]

The reason is that values can be resolved into instances of sympy.Basic. For instance, repr(cirq.ParamResolver({'a': sympy.Symbol('b') + 1})) evaluates to "cirq.ParamResolver({'a': b + 1})" where the b is not a defined expression.

This is currently blocking JSON serialization of ParamResolver because the serialization test insists that the repr be evaluatable. Should we drop this requirement for ParamResolver? Or should we actually try to get an evaluatable repr for arbitrary sympy.Basic instances? As @mpharrigan mentioned in #2014 we could maybe use sympy.srepr.

@Strilanc

[kevinsung]

#2554 fixes this for cases handled by cirq._compat.proper_repr. I think that's the best we can hope for.

[mpharrigan]

Shouldn't JSON serialization be independent of repr-roundtrippability?

[kevinsung]

Maybe, but #2527 made them dependent.