Bug 20550

In long term we want to move from Node.Cookie getCookie methods and replace them
by universal Lookup. Something like that has already been done in Looks and
Actions APIs.

During the usage I have realized that it is relatively hard to create a lookup
instance. Especially for simple case like immutable lookup of single existing
object(s). One can use 

InstanceContent ic = new InstanceContent ();
ic.add (singleton);
return new AbstractLookup (ic);

but the code is long and the memory representation of such object is overkilling
the actual need (because the lookup will never change).

That is why I suggest to create factory methods:

public static Lookup singleton (Object objectToLookup);
public static Lookup fixed (Object[] objectsToLookup);
public static Lookup fixed (Object[] keys, InstanceContent.Convertor convertor);

Or something like this with memory representation as small as possible.