Use IIFE for function/method/class definition

[denis-migdal]

Currently, Brython defines functions as :

function foo288943(){
      var locals
      var locals_exec_foo = locals = {};
      if(arguments.length !== 0){
        foo288943.$args_parser(foo288943, arguments)
      }
      var frame = ["foo", locals, "exec", locals_exec, foo288943]
      $B.enter_frame(frame, __file__, 1)
      try{
        $B.js_this = this
        $B.set_lineno(frame, 2)
        void(0)
        return $B.trace_return_and_leave(frame, _b_.None)
      }catch(err){
        $B.set_exc_and_leave(frame, err)
      }
    }
    foo288943.$function_infos = ['exec', 'foo', 'foo', __file__, _b_.None, _b_.None, _b_.None, [], null, null,
    0, 1, 3, [], 0, 0, [], false, []]
    foo288943.$args_parser = $B.make_args_parser_and_parse
    foo288943.__annotate__ = _b_.None
    locals_exec.foo = foo288943

I think this can be slightly improved by using the IIFE pattern:

locals_exec.foo = (() => {
     function foo() {
         // foo() impl.
     }
     // do some init.
     return foo;
})();

JS engine would immediately inline such function call, so it shouldn't have any cost. However it'd have several advantages:

• you'll be able to have a local context, so won't leak any temporary variables used for the function definition (if needed).
• it will be easier to see where the function definition ends.

TBF I find function definitions quite long, and it would be great to have something like:

local_exec.foo  = $B.createFunction(
       {...}, // function meta-data used to create the function.
       () => {
             void(0)
             return $B.trace_return_and_leave(frame, _b_.None)
       }
);

Call performances should remain identical as I assume JS engines are clever enough to inline the callback.
Function creation may be a little slower... though on the other hand we'll also gain time:

• for JS code generation (as well as JS code storing operations) ;
• for JS code parsing by the browser (which is hard to measure but can be quite slow).

Dunno if it is something worth exploring.