[m-rev.] for review: color schemes

[Zoltan Somogyi]

This diff implements the rough consensus we reached about
a month ago on m-rev on how colors should be controlled,
or at least my understanding of it. For review, preferably,
by everyone, not just by one person.

Peter, would you like to pick the colors to use for the
four builtin color schemes, {dark,light}{16,256}? And do you
have a proposed set of default colors, for use when the user
does not specify a color scheme, and therefore we don't know
what color background his/her terminal has?

Note that I will write the documentation on how users can control
color only after we agree on the details of the mechanisms involved.

-----------------------------------------------------------

I have gone through all the .err_exp files in tests/invalid,
and got the compiler to the state where I now like both the wording
of the diagnostics in those files, and how they are colored.
(In a few cases, such ho_default_func_1, I can still see how
adding more detailed info about the error would be an improvement,
but that is a different topic.)

Over the next few weeks, I intend to go through invalid_nodepend,
invalid_make_int and the other invalid_x and warnings test directories,
doing the same thing. I expect it to faster, both because they contain
fewer tests, and because many of the diagnostics they test for
also occur in tests/invalid, so I already find their formatting
to be ok. But while I do that, you may wish to have look at
tests/invalid/*.err_exp yourself, and see if you can find ways to
improve those diagnostics that I missed.

Zoltan.
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