logger: fix problems with level checking, add option to `log-message`

Logged messages could get dropped if a log receiver is specialized
to a name that is provided as an argument to `log-message` instead
of used from the target logger.

Add an option to `log-message` to avoid adding the name to the
start of the message string, which is needed to propagate messages
from one logger to another.
compiler/cm: fix propagation of log messages
compiler/cm: doc repair
raco make: improve parallelism

The `require` macro now logs "prefetch" messages when it sees a `require`
with multiple module paths. The prefix information is approximate, since
parsing a `require` subform might depend on imports from a previous subform,
but in the common case, there are many obvious module paths to prefetch.

The parallel mode of `raco make` watches for prefetch messages and
records the suggested "prefetch" paths so they can be compiled by other
processes.