Review ImmutableSet method naming

[sharwell]

The map from IImmutableSet<T> to ImmutableSet<T> does not completely align with the Set<T> interface. While some of the names could be updated in the Java code to immediately match the Set<T> conventions, there are many methods which have no direct mapping. It's not currently clear to me how these should all map.

IImmutableSet<T>	ImmutableSet<T>	Set<T>
Intersect	intersect	retainAll
Except	except	removeAll
SymmetricExcept	symmetricExcept	n/a
Union	union	addAll
SetEquals	setEquals	equals
IsProperSubsetOf	isProperSubsetOf	n/a
IsProperSupersetOf	isProperSupersetOf	n/a
IsSubsetOf	isSubsetOf	n/a
IsSupersetOf	isSupersetOf	containsAll