* Refactored unary elementwise ops into a single interface
that is adaptable to elementwise ops with arbitrary number of inputs.

* Renamed EmitUnaryElementwise -> EmitElementwise.
Implemented first binary elementwise op (Power).

* Refactored some of the boiler plate code for emitting cuda kernels to nvrtc
out of the emit functions and into the CudaFunctionPool static singleton.
CodeWriter now saves cuda kernels to ./gpu_codegen.

* Added ops Divide, Subtract & Sign to the GPU transformer.
Subtract and Sign both use custom device helper functions which
have math kernels defined for the op in gpu_cuda_kernel_ops.hpp,
and which are built by a new get_device_helper function.