Verify the functional deserialisation pattern

[chambart]

Something like that:

type buffer = { buffer : string; offset : int; length : int }
let read_int32 buf =
  if buf.offset + 4 >= length then raise Some_error
  else
    let result = read_int32_from_string buf.buffer buf.offset in
    { buf = buf with offset = buf.offset + 4 }

Successions of read shouldn't allocate too much. This is a pattern heavily used in cstruct generated code.

[mshinwell]

Maybe add something to the asmcomp/ testsuite.