Think about using `const` as much as possible in Torbutton code
While working on legacy/trac#24309 (moved), the idea got brought up to use const
as much as possible in Torbutton code (see: https://github.com/arthuredelstein/torbutton/commit/3a1aa3ff006b3f2d2e49931c71f9ec2661143192 for an actual patch for the circuit display).
We should summarize the pros and cons for this idea and then make a decision on what to do and do it.