refactor: make [Cohttp.Body.drain] private (#1065) #1586

Job	Run time
build-test-default (ubuntu-latest, ocaml-variants.4.14.2+options,ocaml-option-afl, *.opam !cohttp...	3m 3s
build-test-default (ubuntu-latest, ocaml-variants.5.02.0+options,ocaml-option-afl, *.opam !cohttp...	7m 9s
build-test-cohttp-bench (ubuntu-latest, 5.x, *.opam )	7m 21s
build-test-default (macos-latest, ocaml-variants.4.14.2+options,ocaml-option-afl, *.opam !cohttp-...	2m 4s
build-test-cohttp-async (ubuntu-latest, ocaml-variants.4.14.2+options,ocaml-option-afl, *.opam !c...	2m 59s
build-test-default (macos-latest, ocaml-variants.5.02.0+options,ocaml-option-afl, *.opam !cohttp-...	5m 30s
build-test-cohttp-async (ubuntu-latest, ocaml-variants.5.02.0+options,ocaml-option-afl, *.opam !c...	8m 55s
build-test-cohttp-eio (ubuntu-latest, 5.x, http.opam cohttp.opam cohttp-eio.opam )	3m 58s
build-test-cohttp-async (macos-latest, ocaml-variants.4.14.2+options,ocaml-option-afl, *.opam !co...	2m 8s
build-test-cohttp-eio (macos-latest, 5.x, http.opam cohttp.opam cohttp-eio.opam )	4m 3s
build-test-cohttp-async (macos-latest, ocaml-variants.5.02.0+options,ocaml-option-afl, *.opam !co...	6m 34s
	53m 44s

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