io: Missing methods on the `Seek` and `BufRead` trait

[Conaclos]

I am in the process of adapting some code from std::io to embedded-io and I noticed some missing methods on mbedded_io::Seek/mbedded_io::BufRead compared to their equivalent std::io:

• Seek::seek_relative
• BufRead::skip_until

Do you plan to add them?
I am specifically interested in BufRead::skip_until and Seek::seek_relative that claims to perform better in some cases.

[Dirbaio]

the goal is for embedded-io to mirror the (stable only) std::io api. So PRs welcome if we're missing things :)