Move start_index out of TranslationResult and into the WasmRuntime.

This makes it more consistent with how all the rest of the content of
a wasm module is handled. And, now TranslationResult just has a Vec
of translated functions, which will make it easier to refactor further.

lib/wasm/src/runtime/dummy.rs

@@ -22,6 +22,9 @@ pub struct DummyRuntime {
 
     // Compilation setting flags.
     flags: settings::Flags,
+
+    // The start function.
+    start_func: Option<FunctionIndex>,
 }
 
 impl DummyRuntime {
@@ -38,6 +41,7 @@ impl DummyRuntime {
             func_types: Vec::new(),
             imported_funcs: Vec::new(),
             flags,
+            start_func: None,
         }
     }
 }
@@ -177,6 +181,11 @@ impl WasmRuntime for DummyRuntime {
         Ok(())
     }
 
+    fn declare_start_func(&mut self, func_index: FunctionIndex) {
+        debug_assert!(self.start_func.is_none());
+        self.start_func = Some(func_index);
+    }
+
     fn begin_translation(&mut self) {
         // We do nothing
     }