Add missing imports needed for format predicates (Mantis bug 371).

compiler/module_imports.m:
    Don't just scan a few kinds of items, scan all items that contain
    goals and/or terms. This includes the instance items that caused
    mantis bug 371, as well as some others (promises, mutables, and
    type definitions).

compiler/prog_data.m:
    Delete an out-of-date comment.

tests/valid/string_format_in_instance.m:
    Add the Mantis test case under a meaningful name.

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