Don't generate nands on BLIF import

[mdko]

In this commit, I updated blif import to be a little more general, but in so doing, introduced a place where n (nand) nets are created. The n net is a non-primitive in PyRTL, and should only be introduced via a call to nand_synth(), and so issues come up if you import a BLIF that generates a n, and then try to export it to Verilog, for example, where checks are made for unsupported nets.

This PR just converts any n seen during BLIF import into a combination of ~ and &.

[codecov-commenter]

Codecov Report

Merging #401 (750cc70) into development (2d9ab35) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           development     #401   +/-   ##
============================================
  Coverage        90.71%   90.71%           
============================================
  Files               24       24           
  Lines             5989     5989           
============================================
  Hits              5433     5433           
  Misses             556      556           

Impacted Files	Coverage Δ
pyrtl/importexport.py	85.65% <100.00%> (ø)

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