Begin work on no_std support.

This adds no_std support to a bunch of things, but more work is needed.

lib/execute/src/lib.rs

@@ -1,12 +1,8 @@
 //! JIT-style runtime for WebAssembly using Cranelift.
 
-#![deny(
-    missing_docs,
-    trivial_numeric_casts,
-    unused_extern_crates,
-    unstable_features
-)]
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
 #![warn(unused_import_braces)]
+#![cfg_attr(feature = "std", deny(unstable_features))]
 #![cfg_attr(
     feature = "clippy",
     plugin(clippy(conf_file = "../../clippy.toml"))
@@ -28,6 +24,8 @@
         use_self
     )
 )]
+#![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(not(feature = "std"), feature(alloc))]
 
 extern crate cranelift_codegen;
 extern crate cranelift_entity;
@@ -35,6 +33,9 @@ extern crate cranelift_wasm;
 extern crate memmap;
 extern crate region;
 extern crate wasmtime_environ;
+#[cfg(not(feature = "std"))]
+#[macro_use]
+extern crate alloc;
 
 mod execute;
 mod instance;
@@ -42,3 +43,10 @@ mod memory;
 
 pub use execute::{compile_and_link_module, execute};
 pub use instance::Instance;
+
+#[cfg(not(feature = "std"))]
+mod std {
+    pub use alloc::{string, vec};
+    pub use core::*;
+    pub use core::{i32, str, u32};
+}