Although "macOS" is the correct name for Apple's current desktop OS,
we've decided to go with "Mac OS" to cover all of Apple's Unix-like
desktop OS versions. The label "Mac OS" is more readable, clear in
context (i.e., unlikely to be confused with the Mac OSes that
proceeded Mac OS X), and as likely to match Apple's future OS names
as anything.
`cext-lib` contains much of the contents of `dynext`, which
is no longer very widely used.
Also moved the implementation of the `mzc` executable
to a more appropriate package.
Also, used `lazy-require` consistently for dynamically
loading implementations.
