make Statement, StatementNoCaseNoDefault and Declaration use SumType

[WebFreak001]

old Declaration code used algebraic, which is more efficiently replaced with sumtype.

I replaced the mixin-generated properties with manual properties, since the code is very small for them anyway and helps them being shown in DCD

Since SumType works with well-defiend types, opEquals and visitIfNotNull was trivial to implement.

Saves around 0-3 MB of actual peak RAM in a ~170 MB test environment (could just be fluctuations how much it actually is, but it will definitely include some RAM savings, since we no longer store a bunch of nulls for common types)

Easy backwards-compatible improvement I think.

Made setters backwards-incompatible on purpose, since we don't want accidental assignment of 2 different types on the same sumtype.

[codecov[bot]]

Codecov Report

Merging #489 (7f7a9ce) into master (98bf0f4) will increase coverage by 0.04%. The diff coverage is 82.27%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #489      +/-   ##
==========================================
+ Coverage   83.49%   83.53%   +0.04%     
==========================================
  Files          11       11              
  Lines        8483     8541      +58     
==========================================
+ Hits         7083     7135      +52     
- Misses       1400     1406       +6     

Impacted Files	Coverage Δ
src/dparse/ast.d	71.97% <80.82%> (+1.72%)	:arrow_up:
src/dparse/parser.d	91.43% <100.00%> (ø)

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