Fix missing caller substitution in apply_wands

[rgwohlbold]

When applying a magic wand containing generic arguments, generic parameters were identity-substituted, leading to a "local variable not found in consistency check" error. Consider the following code snippet:

fn reborrow<U>(x: &mut U) -> &mut U {
    x
}

fn caller<T>(mut v: T) {
    let _ = reborrow(&mut v);
}

When the borrow expires, the following wand is applied in the caller:

  apply acc(p_Param(old[post1](p_Ref_mutable_snap(_2p, T$0)).s_Ref_mutable_0,
    U$0), write) --*
    acc(p_Param(old[pre0](p_Ref_mutable_snap(_3p, T$0)).s_Ref_mutable_0, U$0), write)

Notice the U$0 which has not been substituted.
This PR introduces caller substitution for apply_wand.
Running the doctests shows that this change eliminates this error category, see https://prusti-stdlib-support.rgwohlbold.de/.