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!