Improve ownership semantics of istream with SourceFile

Much less tortured now, and the abstraction is hidden by istream alone, which feels right.
Minor cleanup

Moving out non-urgent TODOs
Verify and maintain the object IDs for ‘null’ and ‘system’

This commit can be reverted later if it turns out that none of the interpreter code needs it.
Let the context message be a Value, not an integer
Implement and test default methods

Test them with and without inheritance.
‘message’ keyword working but only for defined messages
Default methods working with Value messages

Inherited default methods too.
Simplify. Object’s send becomes dispatch.

Leaving SystemObject alone at first.
Testing self, sender, message, sending, passing
Put the ‘system’ object in the Universe

Test the sorter through the Universe, executing send-expressions.

Also prepare for testing parsing this same way.
Parsing through the interpreter

Fixed a couple of subtle bugs in the SystemObject (wrong sender, missing parser->close).

Also moved programs in TestUniverse to be closer to their test methods.