The Scalar and Point interface do not allow error handling

[K1li4nL]

Given the structure of kyber, the following piece of code often appears:

func (P *point) Add(P1, P2 kyber.Point) kyber.Point {
	E1 := P1.(*point)
...

Unfortunately if P1 is not the right type, this will panic. Allowing to return errors would allow graceful handling of the problem:

func (P *point) Add(P1, P2 kyber.Point) (kyber.Point, error) {
	E1, ok := P1.(*point)
        if !ok {
             /* Handle error */
             return nil,  _some error_
...

[pierluca]

Rationale discussed with @bford :
This is to emulate a generic (in pre-generics Go), panicking here makes sense because it's a programmer error, not input-dependent.

A better (longer term) solution would be to get compile-time errors by adopting generics.