Restrict access to private fields

[JSAbrahams]

Relevant issues

Fixes #173

See #188 for an identified issue.

Summary

Private fields (both direct and defined in tuples) and functions cannot be accessed outside a class.

Added Tests

• Happy Access field of a returned object of a function
• Happy Assign to a mutable field of a returned object of a function
• Happy Reassigning to mutable field in chain of calls to mutable objects
• Ignored Sad Assign to an mutable field of a returned object which is an immutable field of self returned by a function
• Sad Assign to an immutable field of a returned object which is a mutable field of self returned by a function
• Sad Assign to an immutable field of a returned object which is an immutable field of self returned by a function (see #188)
• Sad Assign to function call
• Sad Type definition with private field (interface cannot have private fields)
• Sad Type definition with private function (interface cannot have private function)
• Sad Accessing private field of object
• Sad Accessing private function of object
• Sad Accessing private field inside tuple of object
• Sad Reassigning to immutable field in chain of calls with an immutable object raises Type error

[JSAbrahams]

Should add tests that test whether private tuples are indeed private, including their fields after #185 has been merged.