Proposal: make asdf output go to *standard-output*

Hello,

I'd like to propose that asdf send its output to *standard-output* (by
having asdf:operate bind asdf::*verbose-out* to cl:*standard-output*
instead of *trace-output*), and that it be documented that .asd files
and asdf extensions that print output should do so to
*standard-output*, too.  A patch to this effect is appended.

Here's the rationale: there has be some way to stifle output during
asdf:operate in order for programs to use asdf in contexts where it's
necessary to be persnickety about output (e.g., Unix-style scripts and
cron jobs).  At present, there's no way to do this without digging
into asdf.lisp itself, and then there are various deficiencies:

(0) asdf:operate's verbose keyword doesn't stifle very much output
    when nil is supplied.  Compiler and loader output, as well as any
    compile-time or load-time print operations in .asd files or in
    system components still get out.

(1) asdf:operate's verbose keyword only stifles output from that call
    to operate -- output will still be emitted if asdf:operate is
    called during the operation without :verbose nil.  This is the de
    facto standard way to require prerequisite asdf systems, and there
    are a several incidences of this in the wild.

(2) The stream that asdf uses for output internally, *verbose-out*, is
    not exported, and so it's unclear where systems and extensions are
    supposed to send their output.  There's at least one system
    available that prints to *standard-output* in the .asd file.