None of the changes to code gen can actually be tested without
`-instantiate`, which won't be implemented until PR oxcaml#1905. This lets PR oxcaml#2177
just be about typechecking, which we _can_ test immediately (in that we can
check that if you compile with `-as-argument-for` then you have to conform to
the parameter type and we remember that you passed the flag).

For each runtime parameter `P`, this generates a Lambda binding

  let P = raise (Invalid_argument "-parameter not yet implemented")

This, of course, makes the module unusable when linked, but we can still test
that compilation goes through. (Also, linking directly against a parameterised
module _should_ raise an error!)

This effectively reverts the changes from the following commits:

* e8abb3a
* 1cee4f4
* 814a8a7
* 95e7772

Mostly these changes just dropped code gen or added stub code, so we're happy to
revert them. The last one did add some tests, so I'll be un-reverting it.

The test currently fails, as linking shouldn't be allowed against a
parameterised module. (The module will harmlessly load and do nothing, since
it's just a block with a function, but silent failure is bad.)

A partial cherry-pick from commit 95e7772.

Removal of code gen from this branch of parameterised libraries means the list
of runtime parameters is no longer stored in the .cmo (yet).