Parse 61131-3 Classes

[ulmer-a]

After this PR, rusty will be able to parse classes. Closes #219. Please merge PR #211 first.

Note that I accidently rebased the changes from PR #211 into this branch (first 10 commits below). Would have been better to merge it... It's going to be squashed anyway, though.

Deviation from the standard: We do not support VAR_EXTERNAL constructs since they are rarely used. Namespaces, Inheriting and interface have not been implemented, yet.

[codecov]

Codecov Report

Merging #225 (7e44d5a) into master (7e18300) will increase coverage by 0.06%.
The diff coverage is 98.87%.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
+ Coverage   93.52%   93.58%   +0.06%     
==========================================
  Files          34       34              
  Lines        9054     9173     +119     
==========================================
+ Hits         8468     8585     +117     
- Misses        586      588       +2     

Impacted Files	Coverage Δ
src/index.rs	94.40% <ø> (ø)
src/index/visitor.rs	96.18% <0.00%> (-0.37%)	⬇️
src/lexer.rs	95.32% <ø> (ø)
src/parser.rs	97.86% <99.42%> (+0.27%)	⬆️
src/ast.rs	87.83% <100.00%> (+0.05%)	⬆️

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[ghaith]

Just some discussion points

[ghaith]

I think we just need a test for the FB case. Otherwise this looks good. I'll test it a bit locally though before I give the last approval

[ghaith]

What I think might also be good are index tests where we verify instances of a class are initialized correctly, and that we can find methods on classes.

[ulmer-a]

Not entirely sure if the following tests are correct, but I think so