Improve constant expression evaluator

[blitz-1306]

Tasks

This PR resolves #224 and fixes #227.

Changes

• Tweak logic for better constant expression evaluation (isConstant(), evalConstantExpr()):
  • respect file-level constants;
  • support more node types (MemberAccess, IndexAccess VariableDeclaration);
  • support more cast operations (string <-> bytes, string / bytes -> fixed bytes, uint <-> fixed bytes);
• Tweaked existing unit test to respect changes.
• Introduced integration test to have an ability to check against real-world Solidity examples.

Notes

• API changes:
  • isConstant() and evalConstantExpr() are now also accepting VariableDeclaration as an input node.
  • Value type now also includes Buffer (a breaking change).
• Due to breaking changes we would have to bump major version of the package.

Regards.

[codecov-commenter]

Codecov Report

Merging #225 (b9b54ff) into master (f467859) will decrease coverage by 0.16%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
- Coverage   91.81%   91.65%   -0.16%     
==========================================
  Files         267      267              
  Lines        6546     6592      +46     
  Branches     1319     1342      +23     
==========================================
+ Hits         6010     6042      +32     
- Misses        283      288       +5     
- Partials      253      262       +9     

Files	Coverage Δ
src/types/eval_const.ts	86.27% <75.00%> (-4.16%)	⬇️

... and 1 file with indirect coverage changes

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[cd1m0]

Good work! Just nits