lib.fifo: fix reset handling of asynchronous FIFOs.

[jfng]

Asynchronous FIFOs do not properly handle a write domain reset (i.e. w_level is not reset to 0). This PR attempts to fix their behaviour.

[codecov[bot]]

Codecov Report

Merging #591 (0dcf329) into master (f7c2b94) will increase coverage by 0.07%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #591      +/-   ##
==========================================
+ Coverage   81.50%   81.58%   +0.07%     
==========================================
  Files          49       49              
  Lines        6461     6467       +6     
  Branches     1287     1288       +1     
==========================================
+ Hits         5266     5276      +10     
+ Misses       1007     1005       -2     
+ Partials      188      186       -2     

Impacted Files	Coverage Δ
nmigen/lib/fifo.py	94.25% <100.00%> (+0.16%)	:arrow_up:
nmigen/build/run.py	22.05% <0.00%> (ø)
nmigen/sim/_pyrtl.py	97.00% <0.00%> (+1.19%)	:arrow_up:

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