syntax compatibility between do blocks and stmt blocks

See the section `do notation` in the manual for more info.

* nkMacroStmt has been removed
   Macro statements are now mapped to regular nkCall nodes.
   The support for additional clauses (such as else, except, of, etc)
   have been restored - they will now appear as additional arguments
   for the nkCall node (as nkElse, nkExcept, etc nodes)

* fixed some regressions in the `is` operator and semCompiles

lib/impure/graphics.nim

@@ -22,7 +22,7 @@ type
 
   PSurface* = ref TSurface ## a surface to draw onto
   TSurface* {.pure, final.} = object
-    w*, h*: int
+    w*, h*: Natural
     s*: sdl.PSurface
   
   EGraphics* = object of EIO