Broadcast group with non-broadcast items generates ill-typed AIR

[matthias-brun]

broadcast use checks that the argument is a broadcast function but broadcast group seems to be missing that check.

use vstd::prelude::*; verus!{

proof fn lemma_foo()
    ensures true
{}

broadcast group group_foo {
    lemma_foo,
}

proof fn lemma_bar() {
    broadcast use group_foo;
}

}

use vstd::prelude::*; verus!{

proof fn lemma_foo()
    ensures true
{}

broadcast group group_foo {
    lemma_foo,
}

proof fn lemma_bar() {
    broadcast use group_foo;
}

}

cc @utaal