Optimize SoleLogics structures

It's finally time to optimize SoleLogics data structures, removing useless abstract types from the hierarchy (e.g., substituting SyntaxTree with a Union of Atom, BooleanTruth and SyntaxBranch) and, most importantly, from fields in other structs (e.g., NO NTuples{N, T}, such us children in SyntaxBranch).

Also, thank you @Tortar for the insightful feedback!

No files or tests are named in the issue; start by locating the type definitions for `SyntaxTree`, `SyntaxBranch`, and related tuple fields in the source code. Check how these are currently exported/used across structs, then compare with linked PR #84 for already-applied changes. Run the relevant Julia test suite to validate behavior is unchanged. Done means the hierarchy and struct fields no longer use the targeted abstract types and tests still pass.

Rédigé par le modèle d'indexation à partir du texte de l'issue.