In the release notes for the next release, mention that pragmas that
specify a predicate or a function now need a pred() or func() wrapper,
and that pragmas without such wrappers now get a warning, which can be
shut up (in that release) with --no-warn-potentially-ambiguous-pragma.
After the release, upgrade the warning to an error. In the release
after that, remove the option, and always generate an error.

Carve several submodules off io.m, as discussed on m-dev on 2021 Aug 23-34
in a thread about get_environment_var_map.

Eliminate the --trail-segments option.
