Better heuristics for QPs

[marcoeilers]

Silicon's heuristics for selecting in what order to use QP chunks when exhaling QPs are somewhat lacking (see issue #788).

This PR adds a much more precise kind of heuristics that should help a lot with performance when QPs of the same "shape" (as in, the same number and types of quantified variables) are inhaled and exhaled repeatedly.
E.g., if the QP we're looking to find or remove has a quantified variable i: Int and receiver term f(a, i), and an existing chunk with quantified variable x: Int has receiver term f(g(), x), where a == g(), then that chunk would be selected first. We use the decider to check for equality, so it can take all knowledge Silicon currently has into account.
If no such chunk exists, the standard hint based heuristics are used.

[jcp19]

For future reference, my quick and dirty measurements in the master branch of SCION (router package):

• without this PR: 12:14
• with this PR: 12:30
  

[marcoeilers]

Haha :D
For the record, I still think this is a good change. It may cause a tiny bit of additional cost for many cases (but should never be really expensive) and helps massively in some cases.

[jcp19]

I'm not an expert in this code, but the changes look ok to me. Unfortunately, SCION did not get faster with this :'(