Move the async-using parts of benchmarking into a .cpp file

This keeps it out of the main include path when benchmarking is
enabled, somewhat reducing the compilation-time penalty.

Also moved some other functions into the .cpp file, especially
helpers that could be given internal linkage, and concretized some
iterator-templated code that only ever used
`std::vector<double>::iterator`.

scripts/generateSingleHeader.py

@@ -53,7 +53,7 @@ def generate(v):
             out.write( line )
 
     def insertCpps():
-        dirs = [os.path.join( rootPath, s) for s in ['', 'internal', 'reporters']]
+        dirs = [os.path.join( rootPath, s) for s in ['', 'internal', 'reporters', 'internal/benchmark', 'internal/benchmark/detail']]
         cppFiles = []
         for dir in dirs:
             cppFiles += glob(os.path.join(dir, '*.cpp'))