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/sigmatch.nim

@@ -637,8 +637,12 @@ proc ParamTypesMatchAux(c: PContext, m: var TCandidate, f, a: PType,
   of isGeneric:
     inc(m.genericMatches)
     if m.calleeSym != nil and m.calleeSym.kind in {skMacro, skTemplate}:
-      if f.kind == tyTypeDesc: result = arg
-      else: result = argOrig
+      if f.kind == tyStmt and argOrig.kind == nkDo:
+        result = argOrig[bodyPos]
+      elif f.kind == tyTypeDesc:
+        result = arg
+      else:
+        result = argOrig
     else:
       result = copyTree(arg)
       result.typ = getInstantiatedType(c, arg, m, f)