Remove re-declaration of default argument to abc::to_str()

Strangely not caught by GCC, was caught by MSC16.
Preliminary support for non-variadic template implementation of abc::_scope_trace

Move the implementation of abc::_scope_trace<> into a separate class,
abc::_scope_trace_impl, so that the same code can be shared by the
variadic and the non-variadic implementations of abc::_scope_trace.
Add non-variadic template implementation of abc::_stream_trace()

Needs improvement; I couldn’t seem to make SFINAE work for the
constructor, so I had to create N copies of the entire class definition.
Merge remote-tracking branch 'remotes/origin/master' into vs2010
Fix broken abc::char_range_::operator bool()