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/take.rs

@@ -1,6 +1,6 @@
 use crate::Buf;
 
-use std::cmp;
+use core::cmp;
 
 /// A `Buf` adapter which limits the bytes read from an underlying buffer.
 ///