SetMaxEvent allocates storage for all events, which is very expensive
if we want to set a high value; even with small values, the
unnecessary allocation of the initial slice is measurable.

We improve the performance by having a few events preallocated as part
of the trace, and appending as necessary. We also co-locate the flags
in event which makes it smaller (by a word).

We also add a set of benchmarks; before and after amortized cost
per-event is shown:

name                 old time/op  new time/op  delta
Trace/0-2-32         1.19µs ± 1%  0.96µs ± 0%  -18.94%  (p=0.008 n=5+5)
Trace/0-10-32         579ns ± 0%   635ns ± 1%   +9.82%  (p=0.008 n=5+5)
Trace/0-100-32        463ns ± 1%   466ns ± 1%     ~     (p=0.190 n=5+5)
Trace/0-1000-32       451ns ± 1%   451ns ± 0%     ~     (p=0.984 n=5+5)
Trace/0-10000-32      451ns ± 2%   449ns ± 1%     ~     (p=0.492 n=5+5)
Trace/10-2-32        1.41µs ± 1%  0.96µs ± 1%  -31.74%  (p=0.008 n=5+5)
Trace/10-10-32        623ns ± 1%   634ns ± 1%   +1.73%  (p=0.008 n=5+5)
Trace/10-100-32       469ns ± 1%   466ns ± 1%     ~     (p=0.357 n=5+5)
Trace/10-1000-32      452ns ± 1%   453ns ± 2%     ~     (p=0.913 n=5+5)
Trace/10-10000-32     451ns ± 1%   449ns ± 1%     ~     (p=0.175 n=5+5)
Trace/100-2-32       2.78µs ± 2%  0.97µs ± 1%  -65.28%  (p=0.008 n=5+5)
Trace/100-10-32       912ns ± 1%   637ns ± 1%  -30.23%  (p=0.008 n=5+5)
Trace/100-100-32      479ns ± 1%   541ns ± 0%  +12.77%  (p=0.008 n=5+5)
Trace/100-1000-32     510ns ± 0%   541ns ± 1%   +6.08%  (p=0.008 n=5+5)
Trace/100-10000-32    514ns ± 1%   540ns ± 0%   +5.14%  (p=0.008 n=5+5)
Trace/1000-2-32      17.2µs ± 1%   1.0µs ± 1%  -94.39%  (p=0.008 n=5+5)
Trace/1000-10-32     3.90µs ± 1%  0.64µs ± 0%  -83.68%  (p=0.008 n=5+5)
Trace/1000-100-32     814ns ± 1%   542ns ± 0%  -33.46%  (p=0.008 n=5+5)
Trace/1000-1000-32    503ns ± 1%   581ns ± 0%  +15.56%  (p=0.008 n=5+5)
Trace/1000-10000-32  1.28µs ± 2%  1.03µs ± 1%  -19.68%  (p=0.008 n=5+5)

Change-Id: If7c68bb1809fb92fa5d06cb6640be5e09e1f131f
Reviewed-on: https://go-review.googlesource.com/30374
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>