serial interface: add `std::istream` adpater for Arduino `Serial`

[dhebbeker]

Similar to std::cin a global object shall be provided which allows to read formatted input from the serial interface using the Arduino Serial library. This is an interface adapter to 3rd party code.

A name could be serial::cin.

Acceptance criteria:

• a global object is available which fulfils the std::istream interface
• that object can be used to read from serial interface like std::cin

See WIP.

• use std::cin instead of serial_port::cin #88 must be decided first