This fixes at least one "potential" bug in the file
`collects/pkg/private/create.rkt`, where `else` in the
`cond` is bound to `else` from `match` instead of `racket/base`.
(though it turns out that the format will always be
truthy, making the program happen to be correct.)
`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.
