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/buf/vec_deque.rs

@@ -1,4 +1,4 @@
-use std::collections::VecDeque;
+use alloc::collections::VecDeque;
 
 use super::Buf;