Automatically calculate proper read timeout for instructions.

[de-vri-es]

This PR removes the manually provided read_timeout parameter, and instead uses the baud rate of the serial port and the message length to compute the expected response time.

The timing is based on the fact that each byte going over the serial line takes 1 start bit, 8 data bits and 1 stop bit.

The downside is that we now need to query the serial port for the baud rate in Bus::new() and Bus::with_buffers(), so they can return an error.

@omelia-iliffe: As part of this PR I removed serial_port_mut() again. It would allow to change the baud rate of the port, which makes the timeout computation inaccurate. Do you think there is an important enough use case for mutable access to the serial port that we should support it? Note that you don't need mutable access to read/write, only to configure the port.

[omelia-iliffe]

I haven't been through this properly but the only reason I wanted serial_port_mut() is to change the underlying read_timeout. If the timeout is calculated automatically then I don't have any need to it and I can't think of anything else rn.
Looks pretty nice :)

[de-vri-es]

I haven't been through this properly but the only reason I wanted serial_port_mut() is to change the underlying read_timeout. If the timeout is calculated automatically then I don't have any need to it and I can't think of anything else rn.
Looks pretty nice :)

Good to hear 👍

@hgaiser, @wddler: do you think either of you can review this any time soon? No need to go totally in depth, but some sanity checks and another pair of eyes at least :)

[hgaiser]

Not super familiar with this code, but you have my 2 cents :).

[de-vri-es]

Released as v0.9.0 together with the previous changes 🥳