There are use cases like checking hashes of files that benefit from
being able to inspect bytes read as they come in, while still letting
the main code process the bytes as normal (e.g. deserializing into
objects, knowing that if there's a hash failure, you'll discard the
result).

As this is non-trivial to get right (e.g. handling a `buf` that's not
empty when passed to `poll_read`, add a wrapper `InspectReader`
that gets this right, passing all newly read bytes to a supplied `FnMut`
closure.

Fixes: tokio-rs#4584

When writing things out, it's useful to be able to inspect the bytes
that are being written and do things like hash them as they go past.
This isn't trivial to get right, due to partial writes and efficiently
handling vectored writes (if used).

Provide an `InspectWriter` wrapper that gets this right, giving a
supplied `FnMut` closure a chance to inspect the buffers that have been
successfully written out.

Fixes: tokio-rs#4584

Co-authored-by: Alice Ryhl <aliceryhl@google.com>