bencode v0.2.0

Changes:

• make descriptor_table::get_root const qualified.
• rename conversion_error to bad_conversion.
• rename format_json_to to encode_json_to
• throw bencode::out_of_range instead of std::out_of_range
• add accessor functions to bview similar to the accessor functions of basic_bvalue:
  • at(std::size_t)
  • at(std::string_view)
  • operator[](std::size_t)
  • front()
  • back()
• add bpointer: a bencode pointer similar to json pointers.