Support only u64 sized primes for SmallFp

[z-tech]

Though arithmetic of primes 64-128 bits showed some potential at the outset, I don't think the numbers back up supporting this increased complexity especially when the optimizations here can most likely be applied directly to the Fp implementation.

We also have the define_field! macro that chooses the appropriate representation for the user.

Advantages:

• less code to optimize and maintain
• simpler to read and indeed some of the code like multiplication was already kind of duplicated
• we can safely hardcode this lingering const N "number of limbs" to 1, which is kind of a win in itself
• the target primes nowadays are <= u64 anyway

Disadvantages:

• would have been nice to remove before putting in main
• is there a performance win? (I think less so now)

closes: #1089

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[z-tech]

@benbencik