Extensions should be executed within the extended context

[robotlolita]

let Integer-Fib = Integer extended-by: {
  def self fibonacci
    (self === 0) || (self === 1) then: { self }
                                 else: { (self - 1) fibonacci + (self - 2) fibonacci }.
}.

use Integer-Fib in {
  Console write-line!: "Fib 20: " + 20 fibonacci.
}.
# => <Message Send Failure>: <Integer: 19> does not understand fibonacci.

Occurs because (self - n) fibonacci is executed in the global context, not the extended one.