Deprecate let%lwt at the module item level

[aantron]

Using let%lwt as a module item now produces a warning like this:

File "test/ppx/main.ml", line 162, characters 9-41:
162 |          let%lwt result = Lwt.return_true
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning 22: let%lwt should not be used at the module item level.
Replace let%lwt x = e by let x = Lwt_main.run (e)

Resolves #728.

I completely removed support for let%lwt ... and ..., because it was broken. This line

lwt/src/ppx/ppx_lwt.ml

Line 140 in 18e990e

[%expr Lwt.bind [%e pvb_expr] (fun [%p pvar (gen_name i)] -> gen_exp rest (i + 1))]

was accidentally inserting the identifier gen_exp into the output AST, because it is missing an [%e ...] antiquotation, resulting in errors like

File "run_2.ml", line 1, characters 0-49:
Error: Unbound value gen_exp

This means that nobody has ever used let%lwt ... and ... successfully.

I removed the warning that we were trying to print on let%lwt rec ... and ..., because, in this case, the warning seems to be ignored, and the output is simply

File "run_2.ml", line 1, characters 4-7:
Error: Uninterpreted extension 'lwt'.

whether we try to add the warning or not. Given that we will already be discouraging let%lwt at structure item level without and, I think it's fine to have just this uglier message.

The result of these two changes is that we never try to interpret let%lwt ... and ... at all, simplifying the PPX a bit.

[aantron]

(And I checked the blame about the gen_exp issue; the bug has been in the PPX since the beginning.)