Preserve allow_reset_less when remapping ResetSignal. Closes #400

[adamgreig]

See #400

[codecov]

Codecov Report

Merging #401 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
+ Coverage   82.05%   82.10%   +0.04%     
==========================================
  Files          36       36              
  Lines        6075     6075              
  Branches     1234     1234              
==========================================
+ Hits         4985     4988       +3     
+ Misses        915      914       -1     
+ Partials      175      173       -2     

Impacted Files	Coverage Δ
nmigen/hdl/xfrm.py	96.24% <100.00%> (ø)
nmigen/hdl/ir.py	95.47% <0.00%> (+0.25%)	⬆️
nmigen/tracer.py	94.59% <0.00%> (+5.40%)	⬆️

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[whitequark]

Could you add a test please?

[adamgreig]

I've added a test which fails without the change and passes with, though it doesn't explicitly check that the newly created ResetSignal still has allow_reset_less set to True as I wasn't sure how to get at that signal.

[whitequark]

This test is totally fine, thanks!