Always flush stdout to prevent buffering of data

[datdenkikniet]

Thanks for the cool crate :)

This prevents stdout from buffering data internally.

This is relevant when sending binary data (e.g. defmt logs) to stdout, as it will look for some marker to flush (usually \n) which only semi-randomly appears. This causes the data to be buffered internally and flushed as a big block, instead of continually as it flows out of the device, making for a less-than-ideal logging experience.

Our use case is logging over USB with defmt-bbq for the rusty-probe-firmware.

[datdenkikniet]

@JoNil ping! :D (Sorry if it's superfluous, just wanted to make sure you saw the PR).