ast, checker, cgen: fix interface embeded methods call(fix #16496)

[shove70]

1. Fixed Build error: tcc: error: undefined symbol 'color__Color_cprintln', tcc: error: undefined symbol 'color__Brush_cprintln' #16496
2. Add tests.

interface Foo {
    a_method()
}

fn (f Foo) foo_method() int {
    return 0
}

interface Bar {
    Foo
}

fn (b Bar) bar_method() int {
    return b.foo_method()
}

struct Derived {}

fn (d &Derived) a_method() {
}

fn main() {
    bar := Bar(Derived{})
    println(bar.bar_method())
}

outputs:

0

This problem occurs when calls the method of the embedded interface in the interface method, because the transformation mechanism of the interface embedded method is currently missing. This PR implements the same mechanism in the interface by calling the method of the embedded struct from the struct

[spytheman]

Excellent work.

[RGBCube]

Thanks for this fix! I can start working on my coloring library once again...