Custom iterators for lazy evaluation

[Charles-Johnson]

Several places in the codebase could be refactored with a custom iterator to prevent searching down too many paths than needed.

These custom iterators could generate:

• generalisations of a given SyntaxTree
• examples of a given SyntaxTree containing bounded variables
• Concepts that reduce to a given Concept
• sufficient conditions for a given Concept

The current implementation tries to generate the whole set of these up front even though only one valid path is required. The current implementation also doesn't always actually generate every possibility for example applying existential rules only considers examples that directly reduce to the desired normal form and not the other examples that recursively reduce, via rules or implication.

Therefore refactoring the project to use custom iterators will improve performance and let the features handle more complex cases.