Add ec-gpu support

[str4d]

We want to be able to leverage GPUs in halo2, via the ec-gpu crate. This requires two changes:

• The curve implementation needs to support producing the GPU kernel for finite field arithmetic.
• halo2 needs to use GPU-aware MSM and FFT algorithms.

This issue covers the first change. We should implement the GpuField trait from the ec-gpu crate (behind a gpu feature flag), and then the ec-gpu-gen crate should be able to generate the necessary CUDA/OpenCL source code.

[dignifiedquire]

For reference, this is the trait implementation in blstrs

• https://github.com/filecoin-project/blstrs/blob/master/src/lib.rs#L98-L102
• https://github.com/filecoin-project/blstrs/blob/master/src/fp.rs#L659-L672
• https://github.com/filecoin-project/blstrs/blob/master/src/scalar.rs#L613-L626

[dignifiedquire]

Note: currently the MSM and FFT implementations are still in bellperson, ec-gpu-gen just contains the generic curve & field operations.

[str4d]

Closed by #31.