Add buffer method to StdinLock

[ghost]

No description provided.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @dtolnay (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[dtolnay]

Could you provide some context on what kind of usage pattern this would be required in? Ideally do you have some realistic code implementing a realistic use case that requires this method?

What are programs currently doing in the absence of having this method available?

[ghost]

I would have used this to access the buffer without blocking when empty. This usage pattern seems very rare.