Implement Bogoliubov transform as a Gate

[kevinsung]

The main reason I want to do this is to implement ParameterizableEffect (the Givens rotation angles can be parameters) and also eventually MergeableEffect (see quantumlib/Cirq#666). An issue with this is that one wants the decomposition to depend on whether the initial state is a computational basis state or not. Perhaps the computational basis and general version should be separate gates?

[babbush]

I really don't understand the ParameterizableEffect. Can you explain?

[kevinsung]

A ParameterizableEffect is something that can be parameterized by Symbols. In this case, the Givens rotation angles could be Symbols instead of actual values.