Fix missing no_std support in cretonne-module.

And, tidy up the extern crate declarations in the std replacement modules.

lib/module/src/lib.rs

@@ -1,7 +1,8 @@
 //! Top-level lib.rs for `cretonne_module`.
 
 #![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
-#![warn(unused_import_braces, unstable_features)]
+#![warn(unused_import_braces)]
+#![cfg_attr(feature = "std", warn(unstable_features))]
 #![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
 #![cfg_attr(feature = "cargo-clippy", allow(new_without_default, new_without_default_derive))]
 #![cfg_attr(
@@ -11,6 +12,13 @@
         print_stdout, unicode_not_nfc, use_self
     )
 )]
+// Turns on no_std and alloc features if std is not available.
+#![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(not(feature = "std"), feature(alloc))]
+
+#[cfg(not(feature = "std"))]
+#[cfg_attr(test, macro_use)]
+extern crate alloc;
 
 #[macro_use]
 extern crate cretonne_codegen;
@@ -26,3 +34,17 @@ mod module;
 pub use backend::Backend;
 pub use data_context::{DataContext, DataDescription, Init, Writability};
 pub use module::{DataId, FuncId, FuncOrDataId, Linkage, Module, ModuleError, ModuleNamespace};
+
+/// This replaces `std` in builds with `core`.
+#[cfg(not(feature = "std"))]
+mod std {
+    pub use alloc::{borrow, boxed, string, vec};
+    pub use core::*;
+    pub mod collections {
+        #[allow(unused_extern_crates)]
+        extern crate hashmap_core;
+
+        pub use self::hashmap_core::map as hash_map;
+        pub use self::hashmap_core::{HashMap, HashSet};
+    }
+}