[m-rev.] for post-commit review: improve non tail rec warnings from MLDS

[Zoltan Somogyi]

For post commit review by anyone. The diff is with -b.

This implements (most of) what we agreed on on m-rev
about a month ago. I would be interested in people's opinions
on whether the new error message are specific enough,
or whether they should have more detailed info
(about *why* the callee cannot tail-call the caller).

Once I have finished a couple of other related diffs, I plan to add
an entry to NEWS about the MLDS backend now supporting
mutual tail recursion. Should the new option be announced
in NEWS then?


Zoltan.
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