String representation of this enum member.
In the case of regular enums, this is just the name of the member.
In the case of flag enums, it's the names joined by vertical bars, or "None",
if the value is zero.
If an enum's value doesn't match a member's value, the raw value is returned as a string.
Color::Red.to_s # => "Red" IOMode::None.to_s # => "None" (IOMode::Read | IOMode::Write).to_s # => "Read | Write" Color.new(10).to_s # => "10"