cgen: fix array contains method with interface(fix #19670)

[shove70]

1. Fix cgen error: cannot convert 'struct string *' to 'struct string' #19670
2. Add tests.

interface Param {}

const some_strings = ['one', 'two', 'three']

fn test(params ...Param) {
	param := params[0]
	match param {
		string {
			if param in some_strings {
				println('in some_strings')
			}
		}
		else {
			panic('oops')
		}
	}
}

fn main() {
	test('three')
}

outputs:

in some_strings

[spytheman]

Please rebase over master.

[shove70]

Please rebase over master.

OK, it's not fixed yet, I'll rebase it when I fix it. ❤️

[spytheman]

This still fails:

interface Param {}

fn test_is_string_in_array() {
	param := Param('three')
	if param is string {
		if param in ['one', 'two', 'three'] {
			assert true
		} else {
			assert false
		}
	}
}