Allow negative indices on `Logic` and `LogicValue` to relative to the end

[mkorbel1]

Motivation

Python allows you to access the end of a list using negative numbers, which is a nifty feature.

Desired solution

Functions like operator [], getRange, and slice should handle negative numbers, treating it as from the end of the value/signal. -1 is the last element, etc.

Alternatives considered

SystemVerilog does not support this.

[RPG-coder-intc]

Hey Max, I will be working on it.