webassembly/proxy_js: Allow a Python proxy of a function to be undone.

This optimises the case where a Python function is, for example, stored to
a JavaScript attribute and then later retrieved from Python.  The Python
function no longer needs to be a proxy with double proxying needed for the
call from Python -> JavaScript -> Python.

Signed-off-by: Damien George <damien@micropython.org>

tests/ports/webassembly/fun_proxy.mjs

@@ -0,0 +1,52 @@
+// Test proxying of functions between Python and JavaScript.
+
+const mp = await (await import(process.argv[2])).loadMicroPython();
+
+// Create JS functions living on the JS and Py sides.
+globalThis.jsFunAdd = (x, y) => x + y;
+mp.globals.set("js_fun_sub", (x, y) => x - y);
+
+console.log("== JavaScript side ==");
+
+// JS function living on the JS side, should be a function.
+console.log(globalThis.jsFunAdd);
+console.log(globalThis.jsFunAdd(1, 2));
+
+// JS function living on the Py side, should be a function.
+console.log(mp.globals.get("js_fun_sub"));
+console.log(mp.globals.get("js_fun_sub")(1, 2));
+
+mp.runPython(`
+import js
+
+print("== Python side ==")
+
+py_fun_mul = lambda x, y: x * y
+js.pyFunDiv = lambda x, y: x / y
+
+# JS function living on the JS side, should be a JsProxy.
+print(type(js.jsFunAdd))
+print(js.jsFunAdd(1, 2))
+
+# JS function living on the Py side, should be a JsProxy.
+print(type(js_fun_sub))
+print(js_fun_sub(1, 2))
+
+# Py function living on the Py side, should be a function.
+print(type(py_fun_mul))
+print(py_fun_mul(2, 3))
+
+# Py function living on the JS side, should be a function.
+print(type(js.pyFunDiv))
+print(js.pyFunDiv(6, 2))
+`);
+
+console.log("== JavaScript side ==");
+
+// Py function living on the Py side, should be a proxy function.
+console.log(mp.globals.get("py_fun_mul"));
+console.log(mp.globals.get("py_fun_mul")(2, 3));
+
+// Py function living on the JS side, should be a proxy function.
+console.log(globalThis.pyFunDiv);
+console.log(globalThis.pyFunDiv(6, 2));

tests/ports/webassembly/fun_proxy.mjs.exp

@@ -0,0 +1,19 @@
+== JavaScript side ==
+[Function (anonymous)]
+3
+[Function (anonymous)]
+-1
+== Python side ==
+<class 'JsProxy'>
+3
+<class 'JsProxy'>
+-1
+<class 'function'>
+6
+<class 'function'>
+3.0
+== JavaScript side ==
+[Function: obj] { _ref: 4 }
+6
+[Function: obj] { _ref: 3 }
+3