Now we use Cirq backend

[nakatamaho]

We should better use the Cirq as the backend. Consequently, some files are renamed and classes may be modified.

There are at least two advantages to using it instead of direct use of libcustatevec: (i) better programmability since the Cirq is a high level of quantum circuit language and libcustatevec is low level library. (ii) CPU optimization, as well as GPU optimization using libcustatevec, is available for the Cirq.

The following files may be renamed to cirq_bridge or something else.
$ find . -name "cu_q"
./apps/www/lib/cu_quantum.rb
./apps/www/app/controllers/cu_quantum_controller.rb
./apps/www/app/views/cu_quantum
./apps/www/test/controllers/cu_quantum_controller_test.rb

especially, there is a class named CuQuantum in ./apps/www/lib/cu_quantum.rb,
class CuQuantum
and this should also be renamed.

Direct use of libcustatevec will be done in the future when we find very serious problems using Cirq.
Best,
Nakata Maho