I was really hopeful initially that this could be done without
having an `std` feature, but it turns out that `bytes` has a bunch
of special additional integration with `std::io` stuff, e.g.
`std::io::Reader` and `std::io::IoSlice`.

To make the library work as `no_std` we add an `std` feature which
is on by default. When it is off, we compile as `no_std` and make
parts of the API that require `std::io` conditional on the `std`
feature.

per @tarcieri request