Recent versions of ppx_sexp_conv cause warnings in lens code

[frank-emrich]

Compiling Links with the recent version v0.16.0 of ppx_sexp_conv causes many warnings such as the following (which are treated as errors for the purposes of CI)

File "lens/column.ml", lines 4-11, characters 0-23:
 4 | type t = {
 5 |   table : string;
 6 |   name : string;
 7 |   alias : string;
 8 |   typ : Type.t;
 9 |   present : bool;
10 | }
11 | [@@deriving show, sexp]
Error (warning 40 [name-out-of-scope]): Field was selected from type Sexplib0.Sexp_conv_record.Fields.t.

As a temporary workaround, #1178 requires using earlier versions of this library.