Index/IndexMut implementation

[pliniker]

1. Currently they return a &T or &mut T just as a Vec or a slice, but similarly, they will panic on accessing an invalid index.

But Vec and slices are contiguous and the validity of an index is highly predictable. In the Trie, seeing it as a sparse form of a Vec, the validity of the index is highly unpredictable, making a panic very probable. This doesn't seem useful.

The accessors get() and get_mut() are much more practical.

2. unwrap() should be replaced with expect(message) to give clear error messages on panic.

[pliniker]

Also, document the above in the code.

[pliniker]

Ideally, the IndexAssign trait would address the usefulness of using the indexing syntax.

[pliniker]

Implemented (2) but nothing further to be done.