WIP: Optimize findVariable() in no-unused-vars

This is the lowest-hanging fruit - `findVariable()` was taking a significant portion of the overall time due to a linear search of the `variables` array. The first step, with the smallest change to the code, was to add a map of variable names to indices. Look up the variable by name, then get it by its index in the existing list.

This is definitely a work in progress, but I wanted to get feedback before I pursue this further. From this change, I'm seeing a speedup in the profiler from ~20s to <10s.