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/serde.rs

@@ -1,4 +1,4 @@
-use std::{cmp, fmt};
+use core::{cmp, fmt};
 use serde::{Serialize, Serializer, Deserialize, Deserializer, de};
 use super::{Bytes, BytesMut};