fix for issue #9394

    This solves problem of incorrect indexing into vtable
    when method from super trait was called through pointer
    to derived trait.
    Problem was that offset of super trait vtables
    was not calculated at all.
    Now it works, correct offset is calculated by
    traversing all super traits up to super trait
    where method belongs. That is how it is
    intended to work.
auto merge of #9464 : bmaxa/rust/master, r=cmr

I have tried this fix and it seems to work either with single or multiple trait inheritance.

trait Base:Base2 + Base3{
fn foo(&self);
}

trait Base2 {
fn baz(&self);
}

trait Base3{
fn root(&self);
}

trait Super: Base{
fn bar(&self);
}

struct X;

impl Base for X {
fn foo(&self) {
println("base foo");
}

}
impl Base2 for X {
fn baz(&self) {
println("base2 baz");
}

}
impl Base3 for X {
fn root(&self) {
println("base3 root");
}

}
impl Super for X {
fn bar(&self) {
println("super bar");
}
}

fn main() {
let n = X;
let s = &n as &Super;
s.bar();
s.foo(); // super bar
s.baz();
s.root();
}

bmaxa@maxa:~/examples/rust$ rustc error.rs
bmaxa@maxa:~/examples/rust$ ./error 
super bar
base foo
base2 baz
base3 root