`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.
The range of values used to represent "improper lists"
of length 36 to 65 overlapped with the range of values
used to represent other things.
This bug is the new chapion of the "how did we not hit that
earlier?" category. The bug was introduced around v300, at
the latest.
