Explore pulling functionality for tracking offsets into its own unit feature

[bbannier]

With #1074 we recently made %random-access an implicit unit feature so that its code is only active if any related functionality is used. We then e.g., emit additional code to track offsets, or change trimming behavior so that users can set the input position to an earlier position.

Changing the trimming behavior can have noticeable effects on unit behavior as we might now need to hold potentially a lot of data; computing offsets on the other hand is pretty cheap. We should explore whether we can pull the computations for offsets and accessing it via offset into its own unit feature, or even make it available by default. If fact spicy-dump already includes an option to dump offsets with -Q so the code might already be there, or could be unified with whatever we do for a potential offset feature.