Clean up backtrace cleaner

Theoretical perf improvement (see benchmarks/index_v_take_while.rb) plus some refactoring.

NOTE: I removed a method from BacktraceCleaner that had been added to reduce duplication, but I prefer this solution because it clearly exposes the order dependency between assignment to `@inclusion_patterns` and assignment to `@exclusion_patterns` and I don't think the extra method buys us much given that it's name basically duplicates the implementation.