Semantic validation

[riederm]

• adds validation.rs that implements a visitor
• adds example validators (pou, variable)
• adds location support for DataTypeDeclaration

Things to discuss:

• shall we code the visitor by hand or do we switch to a more generic AST-representation? (I think manual is work, but its doable)
• how much state do we need for the specific validators? or should they be pure functions without &self?
• ...

[codecov]

Codecov Report

Merging #201 (6c9086c) into master (18f2db2) will increase coverage by 0.10%.
The diff coverage is 90.15%.

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
+ Coverage   95.10%   95.20%   +0.10%     
==========================================
  Files          35       39       +4     
  Lines        9987    10539     +552     
==========================================
+ Hits         9498    10034     +536     
- Misses        489      505      +16     

Impacted Files	Coverage Δ
src/parser/control_parser.rs	96.15% <0.00%> (ø)
src/typesystem.rs	95.91% <ø> (+0.54%)	⬆️
src/parser/expressions_parser.rs	96.96% <41.66%> (-0.53%)	⬇️
src/validation.rs	80.43% <80.43%> (ø)
src/lexer.rs	95.55% <83.33%> (+0.10%)	⬆️
src/index.rs	94.54% <88.00%> (+0.99%)	⬆️
src/lib.rs	95.93% <88.09%> (+5.65%)	⬆️
src/index/visitor.rs	95.91% <88.88%> (-0.29%)	⬇️
src/ast.rs	90.95% <93.93%> (+0.29%)	⬆️
src/parser.rs	97.81% <95.83%> (+0.08%)	⬆️
... and 13 more

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