enum initialization in var-blocks

[mhasel]

Fixes invalid enum initializers not being validated. I've also reclassified the severity from ImprovementSuggestion to SemanticError. Consider the following example:

PROGRAM prog
VAR
    x : DINT := 12345;
    var3 : (a, b, c) := 7; // this should not just be a warning
END_VAR
    var3 := x; // and neither should this
END_PROGRAM

While we could validate against a literal initializer out of range, doing so for a reference is a different story.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03 🎉

Comparison is base (bcf5f7f) 95.93% compared to head (2f6920e) 95.97%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #904      +/-   ##
==========================================
+ Coverage   95.93%   95.97%   +0.03%     
==========================================
  Files         118      118              
  Lines       38569    38608      +39     
==========================================
+ Hits        37002    37054      +52     
+ Misses       1567     1554      -13     

Impacted Files	Coverage Δ
src/diagnostics.rs	80.13% <100.00%> (+0.02%)	⬆️
src/validation/statement.rs	92.49% <100.00%> (+0.06%)	⬆️
src/validation/variable.rs	91.37% <100.00%> (+1.94%)	⬆️

... and 6 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[riederm]

good job