store only locations, not string values

[bartveneman]

I can think of two major use cases where I think it's most useful to no longer include substrings as aprt of the result:

• Showing the reports on https://www.projectwallace.com/analyze-css
• Storing the analysis report in a database as plain numbers

The reason we don't need or even want to have the strings is that we can always get them on demand when we need them. For the webpage for example, we can get the string values only for the items we are rendering on screen and leave out the rest. This will lead to a messive memory reduction, making analysis and displaying a bunch faster.