More detailed benchmarks
Slight memory reduction

The numbers are too small to see significant impact, but not having a
separate variable for the current time and copying it in reduces the
memory requirements just slightly, so I'll take it.

benchmark                             old ns/op    new ns/op    delta
BenchmarkNoDuration                           6            6   -0.66%
BenchmarkNoDurationDeferred                  91           90   -0.88%
BenchmarkNoDurationNotDeferred               19           20   +2.56%
BenchmarkMSDuration                         309          288   -6.80%
BenchmarkMSDurationDeferred                 428          398   -7.01%
BenchmarkMSDurationNotDeferred              342          322   -5.85%
BenchmarkNSDurationRef                      327          339   +3.67%
BenchmarkNSDurationWarningNotFired          873          869   -0.46%
BenchmarkNSDurationWarningFired            1516         1485   -2.04%
BenchmarkNSNoWarningDurationFired           548          530   -3.28%

benchmark                            old allocs   new allocs    delta
BenchmarkNoDuration                           0            0     n/a%
BenchmarkNoDurationDeferred                   0            0     n/a%
BenchmarkNoDurationNotDeferred                0            0     n/a%
BenchmarkMSDuration                           3            3    0.00%
BenchmarkMSDurationDeferred                   3            3    0.00%
BenchmarkMSDurationNotDeferred                3            3    0.00%
BenchmarkNSDurationRef                        2            2    0.00%
BenchmarkNSDurationWarningNotFired            8            8    0.00%
BenchmarkNSDurationWarningFired               8            8    0.00%
BenchmarkNSNoWarningDurationFired             5            5    0.00%

benchmark                             old bytes    new bytes    delta
BenchmarkNoDuration                           0            0     n/a%
BenchmarkNoDurationDeferred                   0            0     n/a%
BenchmarkNoDurationNotDeferred                0            0     n/a%
BenchmarkMSDuration                         116           91  -21.55%
BenchmarkMSDurationDeferred                 115           91  -20.87%
BenchmarkMSDurationNotDeferred              115           91  -20.87%
BenchmarkNSDurationRef                      197          197    0.00%
BenchmarkNSDurationWarningNotFired          427          402   -5.85%
BenchmarkNSDurationWarningFired             427          403   -5.62%
BenchmarkNSNoWarningDurationFired           222          198  -10.81%