b2 features for benchmarking are in separate module

This is so that the module could be easily imported in e.g.
project-config.jam and then used for default build, custom build
variants, etc.

bench/Jamfile

@@ -11,15 +11,12 @@ import common ;
 import os ;
 import path ;
 import property ;
+import bench.jam ;
 
 import feature ;
 
 path-constant HERE : . ;
 
-feature.feature bench.option : : free optional ;
-feature.feature bench.launcher : : free optional ;
-feature.feature bench.file : : free optional ;
-
 .BENCH_FILES = [ os.environ BENCH_FILES ] ;
 
 local has_nlohmann_json = [

bench/bench.jam

@@ -0,0 +1,5 @@
+import feature ;
+
+feature.feature bench.option : : free optional ;
+feature.feature bench.launcher : : free optional ;
+feature.feature bench.file : : free optional ;