Support computing layout of RPIT

And some refactoring to make code more type safe.

Checks for `Read::bytes()` on an unbuffered Read type.
The default implementation calls `read` for each byte, which can be very
inefficient for data that’s not in memory, such as `File`.

Considerations which I'd like to have feedback on:
* Currently this lint triggers when `.bytes()` is called on any type
that implements `std::io::Read` but not `std::io::BufRead`. This is
quite aggressive and in and may result in false positives. Alternatives:
* Only trigger on concrete types, not generic types. This does mean that
cases where a function is generic over a `R: Read` and calls `.bytes()`
are not caught by the lint, which could be quite a nasty case of this
bug.
  * Only trigger on an allowlist of stdlib types
* Compromise: Is it possible to make this lint `pedantic` on types that
are not on a allowlist?
* Theoretically, a trait implementation of `Read` could override
`.bytes()` with an efficient implementation. I'm not sure how to add
this check to the lint, and I can't find any cases of this being done in
practice.
* I don't think an automatic fix for this lint is possible, but I'd love
to be proven wrong
* This is my first time contributing to clippy, please let me know if I
did anything wrong

Fixes rust-lang#14087
```
changelog: [`unbuffered_bytes`]: new lint
```