Support negative indexes in Array

Support negative indexes in:

- `Array.Value`
- `Array.HasValue`
- `Array.NotHasValue`

Semantics should be the same as in Python. This feature is useful when you want to check boundary values in the array beginning and end. It becomes especially useful with deprecation of First and Last methods.

Bound checks in corresponding methods should be updated. Currently we check and report `AssertInRange` with `Min = 0, Max = len-1`. Now it would be convenient to report this range only if index is `>= 0`, and to check and report `Min = -len, Max = -1` range if index is negative.

Unit tests and documentation comments for all 3 methods should be updated as well.