Refactor Finfet support code to reduce duplication and support all features

[vaughnbetz]

Problem:

1. The FinFET support code has resulted in duplicate code for various basic subcircuits -- e.g. two similar paths for routing mux implementations, one with bulk and one with finFeT transistors.

2. This also means some circuits do not have a finFET path, and hence do not support finFETs (e.g. Fracturable LUTs).

Proposed Solution:

We should refactor the code so bulk vs. finFET transistor instantiation and area modeling and size choices are all done by low level routines to isolate bulk vs. finFET code. Higher level (per subcircuit) code should only call these low level routines. Ideally this would reduce code size while also allowing finFET support for all subcircuits.

[vaughnbetz]

@Ibrahimai : once your other tasks are done, maybe I can buy you many coffees and convince you to do this for COFFE's good.