Introduce an `elsa-annotate` macro for the type definitions

[Fuco1]

(progn
  (put 'stringp 'elsa-type (elsa-make-type Mixed -> Bool))
  (put 'stringp 'elsa-narrow-types (list (elsa-make-type String))))

could become something like

(elsa-annotate stringp :type Mixed -> Bool :narrow (1 String))