Prio3: Improve circuit specifications

Rework the spec of each Prio3 variant by listing the entire class of
each circuit. Also, list the specification of each gadget in the
appendix.

* Prio3Sum: Use `PolyEval` instead of `Range2`, as this is a much more
  useful gadget to list in the doc.

* Say "variant" instead of "instance".

* Document missing operations on polynomials.

* Modify the constructor for `Field` elements by interpreting negative
  integers as additive inverses. This is necessary in order to allow
  expressing polynomial coefficients in the `PolyEval` circuit
  independent of the field modulus.