Cppo_eval.ml:  make the type [env] an algebraic data type.

This avoids some confusion, as the constructors [`Def] and [`Defun]
were previously used in two different types, namely [node] and [env],
with different arguments.

Here, [args] is never empty, so [n] is never zero,
so this test is useless.

In the definition of [`Ident] in the type [node],
the third parameter used to have type [actuals option]:
it was either [None]
or [Some args] where [args] was a nonempty list of actuals.

It is simpler to let this parameter have type [actuals].
There is no need for an option.

The file test/dune is modified to use more modern rule syntax.
This changes the generated .opam files.

The purpose of this change is to allow negative tests.
`with-accepted-exit-codes` requires dune 2.0.

New positive tests:
  lexical.cppo

New negative tests:
  arity_mismatch.cppo
  applied_to_none.cppo
  expects_no_args.cppo
  already_defined.cppo
  at_least_one_arg.cppo

There was no strong reason for this distinction to exist.

A few new auxiliary functions are isolated: [bind_one], [bind_many].

A few error messages change slightly (they become more uniform).
The expected test output is adjusted in a separate commit.

There was no strong reason for this distinction to exist.

Maybe this is not necessary, but it seems safer.