dsl: issue a warning if defining a case after the default

[FFY00]

Resolves #404

Signed-off-by: Filipe Laíns [email protected]

[codecov[bot]]

Codecov Report

Merging #435 into master will decrease coverage by 0.03%. The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master     #435      +/-   ##
==========================================
- Coverage   81.71%   81.68%   -0.04%     
==========================================
  Files          40       40              
  Lines        6099     6103       +4     
  Branches     1242     1244       +2     
==========================================
+ Hits         4984     4985       +1     
- Misses        934      937       +3     
  Partials      181      181              

Impacted Files	Coverage Δ
nmigen/hdl/dsl.py	98.94% <25.00%> (-0.79%)	:arrow_down:
nmigen/build/run.py	31.25% <0.00%> (ø)

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[whitequark]

Thank you for your contribution. I believe that #1059 will address the underlying issue.