wasmtime: Option to disable parallel compilation (#3169)

* Introduce parallel-compilation configuration switch

* Plumb parallel_compilation config to compilation

* Adjust obj.rs

* Address review

* Fix compilation fail in `cache` crate

* Fix obj.rs

Also remove the now unneeded feature in /Cargo.toml

* fmt

src/obj.rs

@@ -62,6 +62,7 @@ pub fn compile_to_obj(
         },
         tunables.clone(),
         features.clone(),
+        true, // enable parallel compilation
     );
 
     let environ = ModuleEnvironment::new(compiler.isa().frontend_config(), &tunables, &features);