cranelift-wasm: Make FuncEnvironment::translate_ref_func take a FuncIndex

It was previously taking a raw `u32`. This change makes it more clear what index
space that index points into.

cranelift/wasm/src/environ/dummy.rs

@@ -516,7 +516,7 @@ impl<'dummy_environment> FuncEnvironment for DummyFuncEnvironment<'dummy_environ
     fn translate_ref_func(
         &mut self,
         mut pos: FuncCursor,
-        _func_index: u32,
+        _func_index: FuncIndex,
     ) -> WasmResult<ir::Value> {
         Ok(pos.ins().null(self.reference_type()))
     }

cranelift/wasm/src/environ/spec.rs

@@ -456,7 +456,11 @@ pub trait FuncEnvironment: TargetEnvironment {
     }
 
     /// Translate a `ref.func` WebAssembly instruction.
-    fn translate_ref_func(&mut self, pos: FuncCursor, func_index: u32) -> WasmResult<ir::Value>;
+    fn translate_ref_func(
+        &mut self,
+        pos: FuncCursor,
+        func_index: FuncIndex,
+    ) -> WasmResult<ir::Value>;
 
     /// Translate a `global.get` WebAssembly instruction at `pos` for a global
     /// that is custom.