Add even-odd benchmark.

Add a new even-odd benchmark that exercises mutual tail recursion.
Note that it is important that both "even" and "odd" are called from
the main program; if only "even" is called, then the Contify
optimization will move both "even'" and "odd'" to their one return
point and turn the tail recursion into a local loop.

A forthcoming MergeTailCalls optimization will merge the "even" and
"odd" functions into single function with two entry points; then
IntroduceLoops will eliminate the self-tail calls in favor of a local
loop; then InlineNonRecursive will (most likely) be able to inline the
resulting non-recursive function.  Even if the function is not
inlined, turning the tail calls into a local loop will be a
performance win.
Update benchmark iteration counts.

Increase the benchmark iteration counts so that all benchmarks run for
at least 15 seconds (for amd64-linux on an "Intel(R) Core(TM) i5-3570K
CPU @ 3.40GHz").

Added an "update-counts.sh" script to calculate appropriate benchmark
iteration counts.

Ideally, the benchmark iteration counts should be loaded dynamically
at the time the benchmark suite is executed (rather than being
hard-coded into the benchmark suite executable).  Then, it would be
easier to calibrate appropriate benchmark iteration counts for
different machines.
Updates to the benchmarking infrastructure.

* bbf87d1 Add an even-odd benchmark to exercise the optimization of
          mutually tail recursive functions
* 34b2958 Increase the iteration counts of benchmarks to obtain
          non-trivial running times.