Add a persistent plan concept

[kknox]

Our library uses the model of an FFT plan; a common idea in the FFT world where an object manipulated by the user represents the 'state' of an FFT operation. The plan also contains references to the OpenCL state that are relevant to the FFT operation, such as the compiled kernel binaries. Some of these operations can be expensive, and it would be nice to allow users to pay the cost once and then serialize the data out to disk, to preserve the work across FFT runs. Further FFT runs could then load the plan state off of disk and continue the operation, hopefully resulting in a performance boost.