Bring prototypical methods inline with the rest of an existing class

[gjtorikian]

Closes https://github.com/atom/biscotto/issues/28.

Previously something like

# Public: Here's a class.
class Foo
  # Here's a method, baz.
  baz: () -> 'baz'

# Public: Here is a method on Foo, called bar.
Foo::bar = () -> 'bar'

would've placed the bar outside of the Foo class. No more! This now bring "File" type definitions into previously defined classes.