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/evals.nim

@@ -1296,7 +1296,7 @@ proc evalAux(c: PEvalContext, n: PNode, flags: TEvalFlags): PNode =
   of nkEmpty: result = n
   of nkSym: result = evalSym(c, n, flags)
   of nkType..nkNilLit: result = copyNode(n) # end of atoms
-  of nkCall, nkHiddenCallConv, nkMacroStmt, nkCommand, nkCallStrLit, nkInfix,
+  of nkCall, nkHiddenCallConv, nkCommand, nkCallStrLit, nkInfix,
      nkPrefix, nkPostfix: 
     result = evalMagicOrCall(c, n)
   of nkCurly, nkBracket, nkRange: