RFR(m): 8177290 add copy factory methods for unmodifiable List, Set, Map

[Stuart Marks]

> Why not using:
>
>     coll.stream().collect(Collectors.toImmutableSet())
>
> As Collectors.toImmutableSet() is currently implemented, with serial Stream it 
> will create a single HashSet, add all the elements to it and call 
> Set.of(HashSet.toArray()) with it. Pretty much the same as what Tagir 
> proposes, but the Collector could be made more efficient in the future and 
> with it, the optimization would automatically extend to Set.copyOf()...
This is mainly about whether Set.copyOf() is implemented in terms of 
Collectors.toUnmodifiableSet(), or vice-versa, which then calls Set.of(T[]) to 
do the actual creation. Some future optimization will probably replace both of 
these implementations with calls to JDK internal methods that can bypass the 
extra copying, so it doesn't really matter which one of these calls the other 
right now.

s'marks