Add no_std support, by adding an std feature (#281)

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.

src/hex.rs

@@ -1,5 +1,5 @@
 use crate::{Bytes, BytesMut};
-use std::fmt::{Formatter, LowerHex, Result, UpperHex};
+use core::fmt::{Formatter, LowerHex, Result, UpperHex};
 
 struct BytesRef<'a>(&'a [u8]);