Export proving key and witness to `websnark` format

[HarryR]

This allows us to use the websnark prover, which seems to be faster than the libsnark based prover built with Emscripten.

For circuits, we would still have to build a WASM version of the circuit generator, but we can make it export the witness in the appropriate format so it can be passed to websnark.

The proving key would need a conversion utility.