message: Add impl for std::string printing,
diff: #include <eosiolib/types.hpp>
  #include <eosiolib/fixed_key.hpp>
  #include <utility>
  +#include <string>
  namespace eosio {
  @@ -21,6 +22,10 @@ namespace eosio {
  prints(ptr);
  }
  + inline void print( const std::string& s) {
  + prints(s.c_str());
  + }
  +
  /**
  * Prints signed integer
  * @brief Prints signed integer as a 64 bit signed integer
  