Specific diagnostic for s/;/:/ typos in du types.

compiler/parse_type_defn.m:
    As above. This prevents du types from defining a :/2 data constructor,
    but since people are much more likely to write that as a typo than
    intentionally, that is no loss.

tests/invalid/du_type_with_colon.{m,err_exp}:
    A test case for the new diagnostic.

tests/invalid/Mmakefile:
    Enable the new test case.