Mapping leaks memory

[gene-pavlovsky]

It would be nice to see Mapping improve one day.
Currently it leaks memory when a same key is added to it.
E.g. this code would result in a mapping that works correctly, but it's underlying list has 4 elements:

		var m:Mapping<String, String> = null;
		m = m.with("a", "1");
		m = m.with("b", "2");
		m = m.with("a", "3");
		m = m.with("a", "4");