cgen, checker: fix option unwrapping and call from option struct field

[felipensp]

Make works:

struct Foo {
mut:
	func ?fn (voidptr) ?bool = unsafe { nil }
}

fn callback(foo &Foo) ?bool {
	return foo.func? == callback
}

fn test_main() {
	t := Foo{
		func: callback
	}
	assert t.func? == callback
	if t.func != none {
		assert t.func(&t)
	} else {
		assert false
	}
}

_{Huly®: V_0.6-21619}

[felipensp]

The CI failure is related to missing fix from PR #23179 (not merged yet)

[spytheman]

Excellent work.