[boost] Removing _1 from the global namespace (again)

If we're going to call the next release 2.0, we'll be allowed some latitude with respect to breaking changes, so maybe this is the right time to finally remove the Bind placeholders from the global namespace. This will, however, break many things all around Boost (to say nothing about the outside world), as a quick grep can readily reveal. And, consequently, it will generate a certain amount of work for the maintainers of the affected libraries that, from their point of view, can appear unnecessary. So I'll need a certain amount of community consensus before going ahead and breaking everyone's code. :-)