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/msgs.nim

@@ -585,22 +585,25 @@ proc inCheckpoint*(current: TLineInfo): TCheckPointResult =
 type
   TErrorHandling = enum doNothing, doAbort, doRaise
 
-proc handleError(msg: TMsgKind, eh: TErrorHandling, s: string) = 
-  if msg == errInternal: 
-    assert(false)             # we want a stack trace here
+proc handleError(msg: TMsgKind, eh: TErrorHandling, s: string) =
+  template maybeTrace =
+    if defined(debug) or gVerbosity >= 3:
+      writeStackTrace()
+
+  if msg == errInternal:
+    writeStackTrace() # we always want a stack trace here
   if msg >= fatalMin and msg <= fatalMax: 
-    if gVerbosity >= 3: assert(false)
+    maybeTrace()
     quit(1)
   if msg >= errMin and msg <= errMax: 
-    if gVerbosity >= 3: assert(false)
+    maybeTrace()
     inc(gErrorCounter)
     options.gExitcode = 1'i8
     if gErrorCounter >= gErrorMax or eh == doAbort: 
-      if gVerbosity >= 3: assert(false)
-      quit(1)                 # one error stops the compiler
+      quit(1)                        # one error stops the compiler
     elif eh == doRaise:
       raiseRecoverableError(s)
-  
+
 proc `==`*(a, b: TLineInfo): bool = 
   result = a.line == b.line and a.fileIndex == b.fileIndex