[boost] [core] Detach demangle() from type_info

Hi Peter, I can see you already extracted demangle() to boost/core. Great, thanks! However I'd prefer it to be detached from type_info. This tool would be useful with different kinds of symbols, like function and variable names, not just type_info. So it better accept a string as an argument and not type_info. demangled_name() should probably be in core/typeinfo.hpp. And regarding typeinfo.hpp. Aren't we duplicating Boost.TypeIndex here? PS: I was also planning to create pull requests to other libraries to use demangle from Core. These are Exception, Units, TypeIndex and Log, AFAIK. Do you plan to do this? If not, I could.