Feature: work on arrays that do not start at 0

[mratsim]

This assumes that indexing starts at 0: https://github.com/numforge/loop-fusion/blob/1b928f224c2c67fc035544c4d870398afd646022/loopfusion.nim#L167-L180

But array can start at an arbitrary value for example: array['0'..'F', uint8] for an array of 8-bit registers or for arrays of enum that goes from -10 to 0