Improve performance of duplicate code detection

[Luro02]

Description

The check is currently one of the slowest checks (it takes up 13.4% of execution time, second slowest check).

The problematic code sections are:

1. countStatements (to decide if a code segment is long enough)
2. Building the occurrences Map (that is related to the implementation of StructuralEqualsVisitor/StructuralHashCodeVisitor).

Here are some ideas to improve performance:

1. Skip adding more kinds of statements in the occurrences map like CtComment
2. Only add multiple statements to the occurrences map like CtStatementList, because a single statement would not be enough for a duplicate? What is with things like if (a) b;?
3. Implement Improve performance of SpoonUtil.resolveConstant #573 which should speed up the isRefactorable method of StructuralEqualsVisitor. It might make sense to build up some cache like with UsesFinder for whether variables are constant/final (I think the whole project would benefit from this).
4. For StructuralHashCodeVisitor the slowest code is the one that calls StructuralEqualsVisitor#shouldSkip, which should become faster with 3