TreeSet/TreeMap should implement Cloneable and offer clone()

[niloc132]

Fixes #10145

[zbynek]

Looks good. There is no speed optimization (it should be possible to clone the map without sorting everything), but that's already noted in

gwt/user/super/com/google/gwt/emul/java/util/TreeMap.java

Line 496 in 0396c65

putAll(map); // TODO(jat): more efficient init from sorted map

[niloc132]

RIght, this doesn't fix that issue at all. I have a draft that tried to fix how we load the data, but it isn't good enough to be worth even putting up a draft yet.