[PATCH] Minor optimisation for Set.copyOf(Collection)

[Andrej Golovnin]

Hi all,

the current implementation of Set.copyOf(Collection<? extends E> coll)
creates an intermediate HashSet object to remove duplicates even when
coll is already a Set. The attached patch adds an additional check
whether coll is already a Set and if yes, then calls #toArray()
directly on coll and passes the result to Set.of().

And when I have already your attention: the constructors of SetN and
MapN may throw a NegativeArraySizeException when the length of the
input is greater than Integer.MAX_VALUE / 2 + 1. Maybe this should be
documented or we should throw IllegalArgumentException and explain
what's went wrong.

Best regards,
Andrej Golovnin