Features to support embedded-io

**What is the name of your project?**

For a cleaner interface separation when writing driver library I want to be able to use `zerocopy` with [embedded-io](https://docs.rs/embedded-io/latest/embedded_io/index.html)-based traits.

**Please provide a link to your project (GitHub repository, crates.io page, etc).**

https://github.com/mauzigoe/tf-luna-rs

Since the project is written for `no_std` Rust I can't use `std::io`. Instead of defining which buffer is expected for my library interface, I just want to pass something which is `Sized` and implements `embedded_io::Read`. Since I use `zerocopy` internally, I can't implement it this way.

**What features would you like from zerocopy?**

Implement a feature which offers functions `read_from_embedded_io` and `write_to_embedded_io` similar to feature `std` in `zerocopy`