Change private bounds from R: Read to R: BufRead

[JonathanBrouwer]

Reasons to do this:

• This matches the stronger bounds of all callers
• Calling these functions with a R that is not a buffered reader would cause performance issues. These functions call .bytes() on the reader which will perform very small reads.
• There will soon be a clippy lint that produces a warning on this code. New lint: unbuffered_bytes rust-clippy#14089
• These are not public functions so this is not a breaking change.

[Byron]

Thanks a lot! That's a great catch and it will prevent the performance issues you mentioned.

It's good these functions aren't public as well, making this PR an easy merge.