Drop todo item.
Start phasing out the usage of the series_tag tag struct.
Build system:
* rework compiler flags setup to be auto-detecting,
* enable certain flags that previously were GCC-only for clang as well, if supported,
* detect the presence of unimplemented type traits at build time and supply substitutes
  in form of boost or GCC-specific traits as needed (via definitions in config.hpp).
series: replace the series_tag struct with is_instance_of in a few more places.