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/function.rs

@@ -1,3 +1,4 @@
+use alloc::string::String;
 use cranelift_codegen::settings;
 use cranelift_codegen::settings::Configurable;
 use cranelift_entity::EntityRef;