This change improves typing support throughout Spina, helping to cement

some patterns, improve code maintenance, and correct typing issues in

derived classes.
SpinaClass is now a generic SpinaClass<TBase>, which takes in the

class being typed, merges with it, and sets the prototype to it. This

replaces the SpinaClass & TBase we had internally in several places,

and gives us a proper up-front type to avoid some casting. Untyped, it

behaves like it did before.
__super__ is now mapped to any instead of object, avoiding type

errors when looking for attributes/functions on parents.
Backbone's _Result<T> is now exported in Spina as Result<T>, giving

us a non-private, stable type we can use when we need to reference a

type or callable returning the type.

Unit tests pass.