[m-dev.] for review: introduction of two new directories

[Zoltan Somogyi]

> > The introduction of two new directories, each containing a new library,
> > requires significant changes to the Mmakefiles in the various directories.
> > I think it would be a good idea for these changes to be reviewed by Fergus
> > and Tyson independently of the other changes in my next commit.
> 
> How about a log message?

The quotes text above has the same information as the log message for
these changes would anyway.

> Why do you need two new directories rather than just one?

One for C code and one for Mercury code.

The Mercury code *could* stay in the standard library, but we already
disable the inclusion of such modules (e.g. debugger_interface) in the
library manual, so they are logically distinct. (The only reason why my
change does not move debugger_interface.m from the library directory
to the browser directory is that I don't want to make difficulties for Erwan;
he can make the move after he commits his outstanding changes.)

Separation has two other benefits: the library shared object is smaller,
and it should eventually be possible to reduce the number of grades in which
the browser needs to be installed. See browser/Mmakefile for details.

Fergus, I thought you knew at least some of these arguments. When I proposed
adding two new directories in an earlier mail, you agreed.

Zoltan.