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

compiler/idents.nim

@@ -101,4 +101,6 @@ proc getIdent*(identifier: string, h: THash): PIdent =
 
 proc IdentEq*(id: PIdent, name: string): bool = 
   result = id.id == getIdent(name).id
-  
+
+let idAnon* = getIdent":anonymous"
+