imageproxy: make .impl_request() generic over fds

...and the presence of a pipe ID.

Create a new internal PipeId structure to keep track of pipe ids on the
server.  This is a very small wrapper around NonZeroU32.

Have .impl_request() return a Result<> of a 3-tuple:
 - a deserialized result, as before
 - a fixed-sized array of fds ([OwnedFd; N])
 - a fixed-sized array of PipeIds ([PipeId; M])

Introduce a bit of generics magic to make sure that we got the expected
number of fds and pipeids from the other side, or an error.

Then you can make calls like:

 - `let ((), [], []) = self.impl_request("CloseImage", [img.0]).await?;`
 - `let (digest, [datafd], [pipeid]) = self.impl_request("GetManifest", [img.0]).await?;`
 - `let (bloblen, [datafd, errfd], []) = self.impl_request("GetRawBlob", args).await?;`

and you'll get an Ok() with the statically-correct number of fds and
pipeids, or you'll get an Err() with a message why, like:

  Expected 1 OwnedFd but got 0

Of course, we can only ever have 0 or 1 PipeIds, but the fixed-sized
array syntax has rather nice ergonomics here and its lets us share logic
with the fds.

Signed-off-by: Allison Karlitskaya <[email protected]>
Signed-off-by: Colin Walters <[email protected]>

Author: allisonkarlitskaya