Make enum members and union tags to reserve top-level type namespace

[dahlia]

In order to make Nirum IDL more portable to various target languages, I added some more rules to restrict name shadowing. See also changelog I wrote.

[codecov]

Codecov Report

Merging #254 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #254      +/-   ##
==========================================
+ Coverage   72.96%   73.13%   +0.17%     
==========================================
  Files          33       33              
  Lines        2341     2356      +15     
  Branches      130      130              
==========================================
+ Hits         1708     1723      +15     
  Misses        503      503              
  Partials      130      130

Impacted Files	Coverage Δ
src/Nirum/Constructs/Declaration.hs	100% <100%> (ø)	⬆️
src/Nirum/Constructs/DeclarationSet.hs	83.92% <100%> (+3.49%)	⬆️
src/Nirum/Constructs/TypeDeclaration.hs	77.85% <100%> (+0.65%)	⬆️

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