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.
And then, finally, we have a fix for defining methods for Result<T>,

without having to suppress errors. Subclasses that define methods for

one or more dynamic Result<T> attributes can now inherit from

DynamicAttrs(BaseClass, attr1, attr2, ...). At runtime, this is a

simple pass-through function that just returns BaseClass. During type

checking, it carefully omits the provided attributes from the parent and

allows the child to define them as methods, which will then be seen in

that class's typing.
This helps avoid all sorts of issues. By default, these attributes are

seen as non-methods, and must be treated as such. But when using

DynamicAttrs, they're seen as methods explicitly, and

subclasses/callers must treat them as such.

Unit tests pass.
Tested this with some in-progress work.