checker, cgen: fix codegen for option fntype

[felipensp]

Fix #22278

[spytheman]

Excellent work.

[spytheman]

There is a small variation, that still produces a cgen error:

type DataFn = fn (name string) string
fn which_lang(name string) string { return name }
fn find_func(name string) ?DataFn {
	a := if name == 'vlang' { ?DataFn(which_lang) } else { none }
	return a
}

edit: filed as #22283 .

[spytheman]

@felipensp should I file a separate issue for it?

[spytheman]

Another (?) cgen error is triggered by:

type DataFn = fn (name string) string
fn which_lang(name string) string { return name }
fn find_func(name string) ?DataFn {
    a := ?DataFn( which_lang )
    return a
}

edit: filed as #22284 .