The motivation is as follows. The sealed trait pattern allows creating
a type-level equivalent of an enum: the trait corresponds to the enum
type and its implementors correspond to the enum variants; the `Sealed`
restriction ensures that there is a fixed set of enum variants.
In this picture, adding methods to the public trait corresponds to a
public method on an enum, while adding methods to the private trait
corresponds to a private method on an enum. This means that we can add
a method to get the basepoint (whose possible choices are enumerated by
SigType) and avoid having to do specialized impls.
This means that using a BindingSig as a SpendAuthSig or vice versa becomes a
compile error. Internally, we can share implementations, but having type
parameters and specialized impls means that the correct parameters can be
substituted in to whatever inner functions exist.
Henry de Valence