Before, `make_channel` assumed that `critical_section` would be
available in the namespace of the code that invoked the macro. Access
`critical_section` through `rtic-sync` instead.
736: More `xtasks` and add examples to `rtic` repo r=korken89 a=datdenkikniet
This was in #732 before, but decluttering that PR seemed sensible
Co-authored-by: datdenkikniet <jcdra1@gmail.com>