Make bench.out in json format

This patch changes the output format of the main benchmark output file
(bench.out) to an extensible format.  I chose JSON over XML because in
addition to being extensible, it is also not too verbose.
Additionally it has good support in python.

The significant change I have made in terms of functionality is to put
timing information as an attribute in JSON instead of a string and to
do that, there is a separate program that prints out a JSON snippet
mentioning the type of timing (hp_timing or clock_gettime).  The mean
timing has now changed from iterations per unit to actual timing per
iteration.

benchtests/scripts/bench.py

@@ -172,7 +172,7 @@ def _print_arg_data(func, directives, all_vals):
 
         # Members for the variants structure list that we will
         # print later.
-        variants.append('  {"%s(%s)", %d, in%d},' % (func, k, len(vals), i))
+        variants.append('  {"%s", %d, in%d},' % (k, len(vals), i))
         print(ARGS_TEMPLATE % {'argnum': i, 'num_args': len(vals),
                                'args': '\n'.join(out)})