Make more code work with no_std. (#407)

* Make more code work with no_std.

no_std support is still incomplete, but this patch takes care of the
bulk of the straightforward parts.

wasmtime-environ/src/lib.rs

@@ -24,14 +24,8 @@
         clippy::use_self
     )
 )]
-#![no_std]
 
-#[cfg(not(feature = "std"))]
-#[macro_use]
-extern crate alloc as std;
-#[cfg(feature = "std")]
-#[macro_use]
-extern crate std;
+extern crate alloc;
 
 #[macro_use]
 extern crate failure_derive;