Eliminate double inverts in pyrtl.optimize

[gaborszita]

Closes #447

Example:

Block before optimize	Block after optimize (without my changes)	Block after optimize (with my changes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.8%. Comparing base (5077c24) to head (6e872b3).

Additional details and impacted files

@@              Coverage Diff              @@
##           development    #462     +/-   ##
=============================================
+ Coverage         91.7%   91.8%   +0.2%     
=============================================
  Files               24      24             
  Lines             6403    6454     +51     
=============================================
+ Hits              5866    5920     +54     
+ Misses             537     534      -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[fdxmw]

Thank you for fixing this issue! A couple high-level comments before we get into coding details:

1. O(N^2) nested for loops over block.logic should generally be avoided in passes because they don't scale well as the number of nets increases - the pass will take too long to run. It seems possible to iterate over block.logic once and build a list of all the inverter chains in the block (see how other passes create a _ProducerList for example). Then I think we can iterate over that list and decide if each inverter chain can be simplified?
2. This really needs a test :) Some test cases that come to mind:
   1. Chains of 1, 2, 3, and 4 inverters (even numbers of inverters should be removed, and odd numbers of inverters should be simplified to one inverter)
   2. A chain of 2 inverters with another user of the first inverter's output (should not be simplified)

[gaborszita]

Thank you for fixing this issue! A couple high-level comments before we get into coding details:

1. O(N^2) nested for loops over block.logic should generally be avoided in passes because they don't scale well as the number of nets increases - the pass will take too long to run. It seems possible to iterate over block.logic once and build a list of all the inverter chains in the block (see how other passes create a _ProducerList for example). Then I think we can iterate over that list and decide if each inverter chain can be simplified?

2. This really needs a test :) Some test cases that come to mind:

   1. Chains of 1, 2, 3, and 4 inverters (even numbers of inverters should be removed, and odd numbers of inverters should be simplified to one inverter)
   2. A chain of 2 inverters with another user of the first inverter's output (should not be simplified)

All addressed