Add way of getting multiple mutable references from storages

Add API that allows getting multiple mutable references from a storage in a way that preserves correctness.
If we had integer generics we could provide generic API:
```rust
fn get_muts(&mut self, [Entity; N]) -> Option<[&mut T; N]>;
```
Right now we could provide more specific instances of it.

The API should be restricted to `DistinctStorage`s and should check that parameter indices are different.