WIP checkpoint. Code that builds.

Use Option<VMTrampoline> which should be ffi-compatible.

Put the register_signature and lookup_signature functions back on the Engine.

lib/api/src/native.rs

@@ -69,6 +69,7 @@ where
             vmctx: other.vmctx,
             signature,
             kind: other.arg_kind,
+            trampoline: None,
         }
     }
 }
@@ -88,6 +89,7 @@ where
                 vmctx: other.vmctx,
                 signature,
                 kind: other.arg_kind,
+                trampoline: None,
             },
         }
     }