Improve circuit optimizer

- [ ] Add buildsystem support (cache optimizer output)
- [ ] Add support for constraints (R1 = R2 * 2)
- [ ] Add support for impedances
- [x] Add example for sizing a sallen key filter topology
- [ ] Add example for sizing a common emitter amplifier

Depends on #21

Review the circuit optimizer context and issue #21 first, since this work depends on it. Use the unchecked checklist as the scope: build-system caching, constraints, impedances, and the common-emitter amplifier sizing example; done means each remaining item is implemented and the examples work.

Written by the indexing model from the issue text.