Provide an alternate implementation of undefined

Recent improvements in the theory of the undefinable suggests
an alternative implementation of undefined which more clearly
expresses the idea that something cannot be expressed.

Acme.Omitted.undefined is a sane alternative to Prelude.undefined,
though it relies on the informal contract that the programmer
never use it to stand for the merely omitted.
Acme.Omitted: distinguish isUndefined and isPreludeUndefined

isUndefined identifies truly undefinable values; isPreludeUndefined
identifies values about which we know nothing more than that
they _are_ undefined.

Also go some way in improving the verbiage that describes these
functions.
Add missing word
Acme.OmittedSpec: make buildable with -Werror

Would fail with -Werror due to type-defaulting.
Fix the problem by giving a specific type.
Write a section on usage
Merge branch 'missing-word'
Merge branch 'fix-test-suite'
Merge branch 'undefined-redefined'

Conflicts:
	tests/Acme/OmittedSpec.hs
Acme.OmittedSpec: use zero in spec for isPreludeUndefined