Benchmarks: Time::Keeper against Time::Piece and DateTime.

From the dist's top, after `perl Makefile.PL && make`, with Time::Keeper::Zone
built beside it (or installed) and DateTime installed:

    perl -Mblib bench/run.pl           # every mark
    perl -Mblib bench/06-strftime.pl   # one

Each line says PASS or FAIL, our rate, theirs, how many times faster, and the
mark set in plan_time_keeper/13-benchmarks-and-gate.md before any number was
seen. A rate is calls a second over a fixed wall-clock time, the best of five.

    TK_BENCH_SECONDS   seconds a run (0.5)
    TK_BENCH_RUNS      runs, the best kept (5)
    TK_BENCH_LOADS     fresh processes for the load time (41)
    TK_BENCH_VALUES    values for bytes per value (200000)
    TK_ZONE_DIST       where Time-Keeper-Zone is built (../Time-Keeper-Zone)
    TK_BENCH_LIB       directories searched last, for modules not installed

DateTime::Format::ISO8601 and Devel::Size are used where installed.
