Add conditional support of Dual Edge FFs

[CheeksTheGeek]

Resolves #164

• implements the first pattern from https://vlsi-soc.blogspot.com/2013/06/dual-edge-triggered-flip-flop.html
  • is only allowed on explicit allowance based on --allow-dual-edge-ff
  • 2 FFs + mux
  • simply put, (posedge ff --> intermediary --> negedge FF --> intermediary) mux selects on these based on clock
  • also an async reset impl for this
  • Made sure to use helpers, similar to canvas->addDff and canvas->addAldff for the dual edge versions

cc: @povik @whitequark

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.85%. Comparing base (f449089) to head (bf83b74).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
+ Coverage   66.17%   66.85%   +0.68%     
==========================================
  Files          18       18              
  Lines        5602     5696      +94     
  Branches     1256     1262       +6     
==========================================
+ Hits         3707     3808     +101     
+ Misses       1486     1483       -3     
+ Partials      409      405       -4     

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[povik]

See comment; I've also pushed a functional test to your branch

[povik]

Please move this to builder.cc and merge it with the async load version