Fix alloc + std builds

There is no reason we shouldn't support both, and we kinda have to
due to rust's features resolution.

Author: TheBlueMatt

src/lib.rs

@@ -39,8 +39,8 @@
 
 #[cfg(any(test, feature="std"))] extern crate core;
 #[cfg(feature="core2")] extern crate core2;
-#[cfg(any(feature = "alloc"))] extern crate alloc;
-#[cfg(any(feature = "std"))] use std as alloc;
+#[cfg(feature = "alloc")] extern crate alloc;
+#[cfg(all(not(feature = "alloc"), feature = "std"))] use std as alloc;
 #[cfg(feature="serde")] pub extern crate serde;
 #[cfg(all(test,feature="serde"))] extern crate serde_test;