[antlr-interest] DebugParser constructors

[Thomas Brandon]

Would it be possible for debug parsers to delegate the public
DebugParser(TokenStream input, DebugEventListener dbg) constructor. So
you can provide a tracing listener, or a listener that doesn't block
on socket. Saves having to keep switching debug on and off when doing
some debugging in AW. Adding through members would break when
debugging wasn't used.

Tom.