use 'using' instead of 'sig' keyword; cleans up new features a bit

tests/misc/tupcomingfeatures.nim

@@ -0,0 +1,39 @@
+discard """
+  output: '''0 -2 0
+  0 -2'''
+"""
+
+{.this: self.}
+
+type
+  Foo {.partial.} = object
+    a, b: int
+
+type
+  tupcomingfeatures.Foo = object
+    x: int
+
+proc yay(self: Foo) =
+  echo a, " ", b, " ", x
+
+proc footest[T](self: var Foo, a: T) =
+  b = 1+a
+  yay()
+
+proc nongeneric(self: Foo) =
+  echo a, " ", b
+
+var ff: Foo
+footest(ff, -3)
+ff.nongeneric
+
+{.experimental.}
+using
+  c: Foo
+  x, y: int
+
+proc usesSig(c) =
+  echo "yummy"
+
+proc foobar(c, y) =
+  echo "yay"