This works within `util.rs`, but fails in `core/backing/src/lib.rs`,
because we don't actually create the struct soon enough. Continuing
down this path would imply substantial rewriting.

This turns out to be relatively complicated and requires some
unsafe code, so we'll want either detailed review, or to choose
to revert this commit.

This means that the work of implementing a subsystem boils down
to implementing the job, and then writing an appropriate
type definition, i.e.

pub type CandidateBackingSubsystem<Spawner, Context> =
	util::JobManager<Spawner, Context, CandidateBackingJob>;

This struct can be constructed when the local node is a validator;
the constructor fails otherwise. It stores a bit of local data, and
provides some utility methods.

The point of making a sub-crate is to ensure that only the necessary
parts of a program get compiled; if a dependent package needed only
subsystem-util, and not subsystem, then subsystem wouldn't need to
be compiled.

However, that will never happen: subsystem-util depends on
subsystem::messages, so subsystem will always be compiled.

Therefore, it makes more sense to add it as a module in the existing
crate than as a new and distinct crate.

Start by moving the JobCanceler here.