convert subsystem-util from crate to module in subsystem

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.