Originally, `Ec::cont` was (afaict) designed to allow switching between threads
in different processes (with different virtual memory space). For switching
between threads inside a single process, it is not needed, but it must be
understood to correctly use `Ec::make_current`, which seems (rightfully)
confusing to many students.

This commit also gets rid of the indirect call of Ec::root_invoke, so now
hopefully it should be easier to understand the program flow during boot.