Proof of concept functions for (mutably) accessing a slice at multiple positions at once via an array of indices.

Example

```rust use index_many::generic::SliceExt;

let mut v = vec![1, 2, 3, 4, 5]; let [a, b, c] = v.indexmanymut([0, 2, 4]); *a += 10; *b += 100; *c += 1000; assert_eq!(v, vec![11, 2, 103, 4, 1005]); ```