Since 5146eb0b (Add support for defer & recover) we have func, which for

    @func
    def f():
        ...

will turn f to be run with additional frame where defer can register calls.

This works ok, but so far the worker of the wrapper was defined inside
func itself - each time func was used, and also the worker had "no
speaking" name _. The latter was making tracebacks a bit harder to read.

-> Move the wrapper to be standalone function with _goframe name. This
removes a bit of import-time overhead when @func is called, and makes
tracebacks a bit more readable.

But my original motivation here is to be able to detect double
func(func(·)) calls and make it idempotent - see next patch for that.

/reviewed-by @levin.zimmermann
/reviewed-on nexedi/pygolang!31