Added simple method to the Serialport struct to open all available ports

[kinire98]

I added this method that would be nice in order to open all the ports in the device. It's pretty simple. I added the documentation. I don't know if the code is all the idiomatic that could be or the docs are right, but I thought this would be useful

[kinire98]

Also, I don't know what's best:

• A vector with a Result for each port (current solution). Can manage each port
• A Result with the vector of ports inside. Returns an error if one is unavailable for some reason

[DanielJoyce]

Since this is so trivial to write, I don't know why this would needed in the public api.