Use a fiber-local instead of a class variable in BenchmarkInterface::BenchmarkSet

* This avoids sharing the BenchmarkSet and also all benchmarks blocks.
  Sharing the benchmark data passed to the benchmark block should be avoided.

Author: eregon

bench/benchmark-interface/lib/benchmark-interface/benchmark-set.rb

@@ -14,7 +14,7 @@ class BenchmarkSet
     def initialize
       @benchmarks = []
       @counter = 0
-      @@current = self
+      Thread.current[:benchmark_interface_benchmark_set] = self
       @iterations = 1
     end
 
@@ -81,10 +81,8 @@ def benchmark(name)
       benchmarks([name]).first
     end
 
-    @@current = nil
-
     def self.current
-      @@current
+      Thread.current[:benchmark_interface_benchmark_set]
     end
 
   end