Move value_t to namespace scope #447

Would you be willing to move 'value_t' into the nlohmann namespace (and not a member of basic_json)? It is independent of the template parameters and would simplify the template instantiations a bit.

I ask mainly because lldb has trouble decoding it. When I break and inspect the stack vars, it will tell me that m_type is "3" but not that it's "string". Moving it outside the templates allows lldb to pierce this and show the actual name of the value.

Although this is more a bug in lldb/clang than a defect in this library, in this case it's a bit cleaner to have non-template things outside the class.