tuple unpacking works in a non-var/let context

tests/tuples/tunpack_asgn.nim

@@ -0,0 +1,32 @@
+discard """
+  output: '''2 4
+4
+2 0'''
+"""
+
+proc foobar(): (int, int) = (2, 4)
+
+# test within a proc:
+proc pp(x: var int) =
+  var y: int
+  (y, x) = foobar()
+
+template pt(x) =
+  var y: int
+  (x, y) = foobar()
+
+# test within a generic:
+proc pg[T](x, y: var T) =
+  pt(x)
+
+# test as a top level statement:
+var x, y, a, b: int
+(x, y) = fooBar()
+
+echo x, " ", y
+
+pp(a)
+echo a
+
+pg(a, b)
+echo a, " ", b