Add BufReader impl

[DaneSlattery]

std::io implements a useful std::io::BufReader.

This is my attempt at implementing for embedded-io.

[DaneSlattery]

This idea spawned from esp-rs/esp-idf-svc#469

[MathiasKoch]

I think this already exists with https://github.com/rmja/buffered-io?

Whether to add it directly to embedded-io is of course another question.

[MabezDev]

I personally don't think this belongs inside embedded-io itself.

I do think it is useful though, and I've personally reached for https://github.com/rmja/buffered-io in my own projects. I think it's a question of

• Is it useful/common enough to warrant living in here?
• If so where should it go, a new utility crate?

This is probably something that needs to be discussed in the WG meeting.

[DaneSlattery]

I think this already exists with https://github.com/rmja/buffered-io?

I see that buffered-io only supports embedded-io-async, perhaps I can retarget this PR towards that crate for the non-async feature?

If so where should it go, a new utility crate?

Perhaps into /embedded-io-adapters?

This is probably something that needs to be discussed in the WG meeting

Let me know what comes of that WG meeting. Happy to make some changes, and perhaps to also replicate some of the ideas from buffered-io, such as passing in the buffer rather than allocating.