make range type checking more restrictive, see tn8vsint16 test case; minor breaking change

tests/range/tn8vsint16.nim

@@ -0,0 +1,18 @@
+discard """
+  output: '''9'''
+"""
+
+type
+  n32 = range[0..high(int)]
+  n8* = range[0'i8..high(int8)]
+
+proc `+`*(a: n32, b: n32{nkIntLit}): n32 = discard
+
+proc `-`*(a: n8, b: n8): n8 = n8(system.`-`(a, b))
+
+var x, y: n8
+var z: int16
+
+# ensure this doesn't call our '-' but system.`-` for int16:
+echo z - n8(9)
+