[m-dev.] promoted ROTDs

[Peter Wang]

Hi,

Continuing on from
https://github.com/Mercury-Language/mercury/issues/38#issuecomment-405848166

The master branch seems to be in a good state right now (as it usually
is), and the start of a month is as good a time as any, so:

Next week, can we start promoting a recent ROTD as a version of choice
for users who can't or don't want to use the "Stable" release, but are
wary of simply picking the latest ROTD in case they happen to hit a
recently-introduced bug (however rare they may be)?

A "preferred ROTD" should also be a useful target for packagers who need
some guidance as to the version to package.

Is there anything known to be wrong with rotd-2018-07-23?
I can do some more testing with that snapshot this week.
I did some testing with a similar version on MinGW-w64 for the snprintf
change. The only test that fails on Linux/x86_64 currently is
tabled_typeclass in debug grades, but AFAIK it should not be blocking.

Then in three months time (or whatever), we can consider promoting a
different ROTD.

Peter