Ref.memoize to defer initialization of Ref'ed value

[JaroslavTulach]

Pull Request Description

• introducing new way to lazily initialize Ref
• such a way can be used to create lazily initialized local variables

v_ref = Ref.memoize <|
   # long computation
   42
v =
    v_ref.get

• only when v is accessed for the first time the long computation is performed
• other changes:
  • moving builtin definitions outside of Ref.enso module
  • into a private module Runtime_Helpers

Important Notes

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• All code follows the
  Java,
• Unit tests have been written where possible.