[WIP] primefield: add "fused" ops to FieldExt

[tarcieri]

Exploring what might make sense here. Starting with:

• add_mul: (a + b) * c
• sub_mul: (a - b) * c

The only field backend where I've done an optimization for these is p521. Notably its multiply takes a LooseFieldElement so there is further potential for another add/sub operation in place of c, e.g. (a + b) * (c + d).

Before going any farther down this road it would be good to look at:

• what works well for the k256 field element implementation
• what we could potentially add to primefield::MontyFieldElement in a generic implementation

[tarcieri]

I'll note whether you do an add/sub-style operation first versus a mul is important to how these fusions would get leveraged in an algorithm like RCB, so I don't think the right answer is we should provide both add_mul and mul_add because e.g. p521 can't optimize mul_add but can optimize add_mul, so it doesn't help if a downstream user thinks mul_add is more convenient only to discover it offers no performance benefits.