Added a missing word in the lambda section of syntax.scrbl
Remove units in parts of `compiler`, `dynext`, `setup` and create the `cext-lib` package.

`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.