BUG-1270: be smart about Restrictions

Restrictions are based on type definitions, which are constant. For base
types they always are. For extended types, they are usually empty.
Optimize memory usage by reusing the lists anchored in types and also by
having a singleton instance for the all-empty case.

Change-Id: Ie9773d7efb57d403a008b82716eb22d97fefb224
Signed-off-by: Robert Varga <rovarga@cisco.com>