109.19.00

- Reworked a number of `Reader` functions to improve performance by
  avoiding deferreds.

  This is a followup to the `Reader` improvements in 109.14, and
  eliminates some last vestiges of performance degradation that had
  been introduced in 109.04.
- Added function `Reader.lseek : t -> int64 -> mode:[< `Set | `End] ->
  int64 Deferred.t`.

  `lseek t offset ~mode` clears `t`'s buffer and calls `Unix.lseek` on
  `t`'s file descriptor.
- Added function `Writer.bytes_received : t -> int`.
- Added function `Unix.mkfifo : ?perm:file_perm -> string -> unit
  Deferred.t`, which was mistakenly missing.

  This is a simple wrapper around `Core.Unix.mkfifo`.