Add Lwt_bytes.{unsafe_,}blit_from_string

[hhugo]

Lwt_bytes was missing bliting from string.

• add blit_from_string and unsafe_blit_from_string
• update doc to make distinction between string and bytes
• add tests for the new functions

[raphael-proust]

Rebased on master where the CI is fixed

[raphael-proust]

I pushed an additional commit which is only style: it replaces the following pattern

try
  let () = Lwt_bytes.… in
  Lwt.return_false
with
| Invalid_argument _ -> Lwt.return_true
| _ -> Lwt.return_true

with the following pattern

match Lwt_bytes.… with
| exception Invalid_argument _ -> Lwt.return_true
| () -> Lwt.return_false

(Also, it adds one test.)

We can roll it back if there are any issues with the test. @hhugo can you have a look at the commit?

[raphael-proust]

Probably the new pattern for tests is not compatible with old versions of OCaml so I'll revert it if the CI indicates so.