stmhal: Overhaul UART class to use read/write, and improve it.
UART object now uses a stream-like interface: read, readall, readline, readinto, readchar, write, writechar. Timeouts are configured when the UART object is initialised, using timeout and timeout_char keyword args. The object includes optional read buffering, using interrupts. You can set the buffer size dynamically using read_buf_len keyword arg. A size of 0 disables buffering.