[m-rev.] for review: diagnose "colon instead of semicolon" typos in du type definitions

[Zoltan Somogyi]

I made this mistake one too many times. I think a diagnostic
in such cases is worthwhile because the difference is hard to see
even for my aging eyes, not just for people with limited vision
like Paul.

Note that this diagnostic is currently an error, but it would be
trivial to make it a warning. However, that warning would be
accompanied by the totally spurious and confusing errors
that we now generate.

For review by anyone.

Zoltan.
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