The uint48 functions are both broken

[talex5]

(reported against Eio in ocaml-multicore/eio#418, which contains fixes and tests)

faraday/lib/faraday.ml

Lines 290 to 297 in c66fc02

	let write_uint48 t i =
	writable_exn t;
	ensure_space t 6;
	Bigstringaf.unsafe_set_int32_be t.buffer t.write_pos
	Int64.(to_int32 (shift_right_logical i 4));
	Bigstringaf.unsafe_set_int16_be t.buffer (t.write_pos + 2)
	Int64.(to_int i);
	t.write_pos <- t.write_pos + 6

@adatario noticed that the shifts should be in bits, not bytes.

Also, the BE version gets the sizes of the chunks wrong (should be 16 then 32).