core 109.38.00

## Async_extra

- In `Rpc`, exposed accessors for binary protocol values.

  For example, this allows one to write a wrapper for `Pipe_rpc` that
  allows for the easy re cording and replaying of values the come over
  the pipe.

## Async_unix

- Added `Reader.of_pipe` and `Writer.of_pipe`, for converting from `string Pipe.t`'s.

  These can be used to add arbitrary transformations (e.g. encryption, compression)
  to code that expects plain file- or socket-based readers and writers.