feat/fix(const-eval,codegen): eval const identifiers to handle const structs

[anton-trunov]

Closes #439

Note: despite the fact constant identifier lookups are supported now, this PR does not resolve issue #229 because a major rewrite of the typechecker (resolveDescriptors.ts) is needed to do that.

• I have updated CHANGELOG.md
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• I did not do unrelated and/or undiscussed refactorings