Clean up backtrace cleaner
Created by: dchelimsky
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.