Clarify guarantees in traits around point validity about algebra

[kobigurk]

Good point! In my opinion, the current methods are useful and needed so I'd leave them as is.
New traits/methods sound like a good idea to me. Ideally they'd be wrappers that check validity and maybe one that multiplies by the cofactor, but that should be marked clearly. That bit us on the gadget side when we migrated our pre-refactor gadgets to the new constrain system.

[Pratyush]

@kobigurk @paberr do you have any thoughts around this? We could have a separate trait that handles the boundary conversions, and leave AffineCurve to be only prime order.

[Pratyush]

One idea would be to put these methods in SWModelParameters and in TEModelParameters (I don't see how to put them in ModelParameters, because the trait doesn't understand the notion of AffineCurve.

[Pratyush]

We now have a new_unchecked which doesn't check for subgroup membership, and have changed new to instead do the on-curve and in-subgroup checks.

(These methods were anyway methods on the structs, and not part of the corresponding traits.)