Fix Record Operators

[awygle]

Proxy Operator calls on Record through to Value implementation, using explicit enumeration to avoid depending on future changes to Value.

Fixes #533 .

[codecov]

Codecov Report

Merging #541 (7ce4830) into master (ebbdac9) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #541      +/-   ##
==========================================
+ Coverage   81.33%   81.41%   +0.08%     
==========================================
  Files          49       49              
  Lines        6461     6462       +1     
  Branches     1289     1289              
==========================================
+ Hits         5255     5261       +6     
+ Misses       1016     1011       -5     
  Partials      190      190              

Impacted Files	Coverage Δ
nmigen/hdl/rec.py	93.33% <100.00%> (+0.59%)	⬆️
nmigen/build/run.py	22.05% <0.00%> (ø)
nmigen/hdl/ast.py	89.62% <0.00%> (+0.42%)	⬆️

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[whitequark]

Thanks!