cgen: fix smartcast for struct fields(fix #20167)

[shove70]

1. Fixed Cgen doesn't unwrap sumtype to another #20167
2. Add tests.

import x.json2

type Foo = f64 | int | string

fn f(j map[string]json2.Any) {
	println(j['name'] or { return })
	assert (j['name'] or { return } as string) == 'foo'
}

struct Bar {
	foo Foo
}

fn g(bar Bar) {
	f({
		'name':  json2.Any('foo')
		'value': match bar.foo {
			string { bar.foo }
			int { bar.foo }
			f64 { bar.foo }
		}
	})
}

fn main() {
	g(Bar{
		foo: Foo(42)
	})
}

outputs:

foo

[spytheman]

Excellent work.