Implement two-adic fields in MSL (Metal)

[xqft]

We need operations over finite fields defined in the Metal Shading Language for implementing FFT (#116) and possibly other future algorithms in the GPU. So it's required to make implementations that mimic the ones we already have in Rust.

Some requirements:

• Basic operations like sum, sub, mul, div.
• Watch out for integer overflow, special care is needed to implement the mul operation for example.
• Ideally we could build a field with a custom module (represented as an uint64_t or whichever type is needed)
• MSL only supports up to 64-bit integers, so it would be needed to also implement classes for bigger ints (specifically see which fields with big integers there are implemented on Rust right now)
• The work directory will be math/src/fft/metal/

Relevant resources:

• MSL Specification (MSL is based on C++)
• The Metal documentation and also see our metal_playground repo for examples on how to use the metal crate in Rust, useful for testing.
• risc0 and ministark have some examples of prime fields implemented in C or MSL.
• Us
• General information about finite fields and modular arithmetic can be easily found on the web.

[xqft]

Done in #193