Add helpers to get 0/1 as integer associated types

[divergentdave]

This adds new methods zero_integer() and one_integer() to FieldElementWithInteger. These replace an existing pattern of constructing zero or one with a round trip through the Montgomery representation, by constructing it as a field element first, and then converting it back. This preparatory work for addressing #643.

I considered and rejected creating a new trait for integers, and putting these methods there, because adding a new trait bound to the Integer associated type would be a breaking change. We could reconsider this API design the next time we make significant breaking changes.

[tgeoghegan]

In the interest of limiting the public API commitment, would it be possible to provide the public helpers in #820 while keeping the new zero_integer and one_integer methods private? i.e. put them on a new trait FieldElementWithIntegerPrivate (a mouthful, I know) that we don't export.

edit: ah, we already have FieldElementWithIntegerExt for just this purpose.

[divergentdave]

Sure, we can always make it public later, maybe via a new Integer trait laid out above