Re: [boost] [OpenMethod] review starts on 28th of April

I'm not sure about BOOST_OPENMETHOD_GUIDE either. It seemed like it was needed so we can add overriding functions to types in a different namespace without opening it. I think this is actually something we don't want to encourage for reasons mentioned by Andrzej about this being a potential footgun.

I added it because we can add a function to a namespace "from outside": namespace foo {} void foo::bar() { ... } ...but it drags users into the internals of the macros. I am probably going to remove it, and document that BOOST_OPENMETHOD_OVERRIDE must be in a namespace in which the method is visible by normal lookup rules.