Simplify traits in utils

[ken-matsui]

This patch simplifies traits defined in utils. Due to the simplicity, this potentially improves compilation performance.

• CanConvertFromStringStream:
  • Use SFINAE.
  • If the std::declval<std::stringstream &>() >> std::declval<T &>() expression is invalid, this specialization will be discarded and fall back to the false_type case.
• CanConvertToString:
  • Likewise.
  • We don't need remove_cvref since std::to_string does not require cv-qualifiers or a ref-qualifier.
• CanConstructFromString:
  • This is equivalent to std::is_constructible<T, std::string>
• CanConvertFromString:
  • This is equivalent to std::is_assignable<T &, std::string>

[an-tao]

@ken-matsui Would u please add some unit tests for this? thanks so much.