The InternMap class provides the canonical value object corresponding to the
specified key. An example:
var map = new InternMap<int, string>(i => i.ToString());
var s1 = map.Intern(123);
var s2 = map.Intern(123);where s1 and s2 refer the same object.
This class has the ConcurrentDictionary class and just wraps its GetOrAdd
methods.