Merge: Make all Sets cloneable

Since there is no reason to forbid it (or I have not seen one at the very least), here's a simple `clone` method that should work on any `Set`

Pull-Request: #1893
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>

lib/core/collection/abstract_collection.nit

@@ -409,6 +409,7 @@ end
 #      assert s.has(b)      ==  true
 interface Set[E]
 	super SimpleCollection[E]
+	super Cloneable
 
 	redef fun has_only(item)
 	do
@@ -471,6 +472,8 @@ interface Set[E]
 		return nhs
 	end
 
+	redef fun clone do return union(self)
+
 	# Returns a new instance of `Set`.
 	#
 	# Depends on the subclass, mainly used for copy services

lib/core/collection/array.nit

@@ -592,7 +592,6 @@ end
 # A set implemented with an Array.
 class ArraySet[E]
 	super Set[E]
-	super Cloneable
 
 	# The stored elements.
 	private var array: Array[E] is noinit

tests/sav/test_new_native_alt1.res

@@ -1,4 +1,4 @@
-Runtime error: Cast failed. Expected `E`, got `Bool` (../lib/core/collection/array.nit:989)
+Runtime error: Cast failed. Expected `E`, got `Bool` (../lib/core/collection/array.nit:988)
 NativeString
 0x4e
 Nit