Bug: field-name associated with dynamic size field will mismatch actual field-name when encoding uses `def`

The description of the encoding of an obj1 (or obj2, etc.) with a named field whose encoding would require a dynamic size field to precede it will under certain conditions ignore the actual name of the field, and instead impute a different name if the encoding of the field is wrapped in a def combinator (may be reproducible under other conditions as well):

Example: (See !50 )

open Data_encoding

let print e =
  let desc =
    Data_encoding.(
      Json.(
        to_string
          (construct Binary_schema.encoding (Binary.describe e))))
  in
  print_endline desc;
  print_newline ();
  flush_all ()

let bugged_encoding = def "actual_name" string
let encoding = obj1 (req "expected_name" bugged_encoding)

let () =
  print encoding

produces

{ "toplevel":
    { "fields":
        [ { "kind": "dyn", "name": "actual_name", "num_fields": 1,
            "size": "Uint30" },
          { "name": "expected_name", "layout": { "kind": "String" },
            "data_kind": { "kind": "Variable" }, "kind": "named" } ] },
  "fields": [] }

In which the field name "actual_name" is used for the dynamic size-field, rather than "expected_name", which is what it ought to produce.