Explore controlled_effect() semantics

[cognigami]

The semantics of quantum_if() might create a little confusion about its purpose[1], so we want to experiment with an expanded approach. If this new approach is successful, the quantum_if() method will probably survive as a shorthand for the expanded approach.

We'd like to add a controlled_effect() method:

• Separate the control conditions, the controlling QuantumObject objects, and the controlled QuantumObject objects.
• Vaguely: controlled_effect(conditions).from(QuantumObject, ...).to(QuantumObject, ...)
• Returns a QuantumEffect

1: Thanks to @dstrain115 for PR https://github.com/quantumlib/unitary/pull/14, which tries to clarify the purpose of quantum_if().