kad: Avoid from/into IteratorIndex.

[twittner]

Type inference failed previously:

error[E0283]: type annotations needed for `(usize, &mut query::peers::closest::ClosestPeersIter)`
   --> /usr/local/cargo/registry/src/gitpro.ttaallkk.top-1ecc6299db9ec823/libp2p-kad-0.20.0/src/query/peers/closest/disjoint.rs:117:22
    |
116 |             for (i, iter) in &mut self.iters.iter_mut().enumerate() {
    |                              -------------------------------------- the element type for this iterator is not specified
117 |                 if i != (*initiated_by).into() {
    |                      ^^ cannot infer type for type `usize`
    |
    = note: cannot satisfy `usize: std::cmp::PartialEq<_>`
error[E0283]: type annotations needed for `(usize, &mut query::peers::closest::ClosestPeersIter)`
   --> /usr/local/cargo/registry/src/gitpro.ttaallkk.top-1ecc6299db9ec823/libp2p-kad-0.20.0/src/query/peers/closest/disjoint.rs:165:22
    |
164 |             for (i, iter) in &mut self.iters.iter_mut().enumerate() {
    |                              -------------------------------------- the element type for this iterator is not specified
165 |                 if i != (*initiated_by).into() {
    |                      ^^ cannot infer type for type `usize`
    |
    = note: cannot satisfy `usize: std::cmp::PartialEq<_>`

[mxinden]

Thanks for fixing this!