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-obj/src/lib.rs

@@ -26,6 +26,8 @@
     )
 )]
 
+extern crate alloc;
+
 mod context;
 mod data_segment;
 mod function;