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/c2nim/cparse.nim

@@ -1382,7 +1382,7 @@ proc nestedStatement(p: var TParser): PNode =
   # Nimrod requires complex statements to be nested in whitespace!
   const
     complexStmt = {nkProcDef, nkMethodDef, nkConverterDef, nkMacroDef,
-      nkTemplateDef, nkIteratorDef, nkMacroStmt, nkIfStmt,
+      nkTemplateDef, nkIteratorDef, nkIfStmt,
       nkWhenStmt, nkForStmt, nkWhileStmt, nkCaseStmt, nkVarSection, 
       nkConstSection, nkTypeSection, nkTryStmt, nkBlockStmt, nkStmtList,
       nkCommentStmt, nkStmtListExpr, nkBlockExpr, nkStmtListType, nkBlockType}