Create a flag_set out of an integral value

[meikjaeckle]

Implements #160

I extended the flag_set template class by a static method which allows to create the flag_set out of an integral value.

This is helpful for e.g. if you read the flags from a device via serial communication.

e.g.

uint8_t readErrorFromDevice();

enum class ErrorBits
{
    ERROR_BIT_0,
    ERROR_BIT_1,
    ERROR_BIT_2,
    ERROR_BIT_3,
    _flag_set_size,
};

using ErrorStatus = type_safe::flag_set<ErrorBits>;

ErrorStatus status = ErrorStatus::from_int(readErrorFromDevice())

[foonathan]

I like it, thank you! Just one comment.

[saburch]

Great addition @meikjaeckle. Very helpful change.
@foonathan Would be nice to have it in an official release 😄

[foonathan]

Thanks! And sorry for the delay.