Introduce `BufRead::skip_until`

[peterkrull]

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I think this standard implementation could be useful for BufRead, based on:

• https://doc.rust-lang.org/src/std/io/mod.rs.html#2180

Because an EOF condition can appear here, I also think renaming ReadExactError -> OperationError to make it more general. Though, that also opens the door for more error variants being added to it, so perhaps it should be #[non_exhaustive]?