Add no_std support

Cargo.toml

@@ -10,7 +10,10 @@ readme = "README.md"
 documentation = "http://doc.servo.org/smallvec/"
 
 [features]
-heapsizeof = ["heapsize"]
+heapsizeof = ["heapsize", "std"]
+collections = []
+std = []
+default = ["std"]
 
 [lib]
 name = "smallvec"

lib.rs

@@ -6,9 +6,24 @@
 //! to the heap for larger allocations.  This can be a useful optimization for improving cache
 //! locality and reducing allocator traffic for workloads that fit within the inline buffer.
 
+#![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(not(feature = "std"), feature(collections))]
+
+
+#[cfg(not(feature = "std"))]
+extern crate collections;
+
+#[cfg(not(feature = "std"))]
+use collections::Vec;
+
 #[cfg(feature="heapsizeof")]
 extern crate heapsize;
 
+#[cfg(not(feature = "std"))]
+mod std {
+    pub use core::*;
+}
+
 use std::borrow::{Borrow, BorrowMut};
 use std::cmp;
 use std::fmt;