Naming conflict with GCC 11

[abougouffa]

I'm trying to compile a project which uses Pangolin, I got an error when compiling with GCC-11:

/usr/include/pangolin/utils/picojson.h:273:65: error: macro "_serialize" passed 2 arguments, but takes just 0
  273 |     template <typename Iter> void _serialize(Iter os, int indent) const;
...
...
/usr/lib/gcc/x86_64-pc-linux-gnu/11.1.0/include/serializeintrin.h:37: note: macro "_serialize" defined here
   37 | #define _serialize()    __builtin_ia32_serialize ()
      | 

In fact, GCC-11 defines a new macro _serialize, creating a conflict with the private member defined in pangolin/utils/picojson.h.

In general, I think it will be safer to name private members as something_ rather than _something since all names starting with an underscore are considered as reserved names in GCC

[stevenlovegrove]

Related to kazuho/picojson#141

Thanks again for this! Your PR is merged.