783: Reimplement BufRead for BufferedUart r=Dirbaio a=chemicstry
The `AsyncBufRead` implementation for `BufferedUart` was removed in https://github.com/embassy-rs/embassy/pull/752, this PR reimplements it from `embedded-io`. This allows reading `BufferedUart` without copying slices.
Co-authored-by: chemicstry <chemicstry@gmail.com>