context: reduce memory usage of context tree

context: reduce memory usage of context tree

Modifies context package to use map[]struct{} rather than map[]bool,
since the map is intended as a set object.  Also adds Benchmarks to
the context package switching between different types of root nodes
and a tree with different depths.

Included below are bytes deltas between the old and new code, using
these benchmarks.

benchmark                                                       old bytes     new bytes     delta
BenchmarkContextCancelTree/depth=1/Root=Background-8            176           176           +0.00%
BenchmarkContextCancelTree/depth=1/Root=OpenCanceler-8          560           544           -2.86%
BenchmarkContextCancelTree/depth=1/Root=ClosedCanceler-8        352           352           +0.00%
BenchmarkContextCancelTree/depth=10/Root=Background-8           3632          3488          -3.96%
BenchmarkContextCancelTree/depth=10/Root=OpenCanceler-8         4016          3856          -3.98%
BenchmarkContextCancelTree/depth=10/Root=ClosedCanceler-8       1936          1936          +0.00%
BenchmarkContextCancelTree/depth=100/Root=Background-8          38192         36608         -4.15%
BenchmarkContextCancelTree/depth=100/Root=OpenCanceler-8        38576         36976         -4.15%
BenchmarkContextCancelTree/depth=100/Root=ClosedCanceler-8      17776         17776         +0.00%
BenchmarkContextCancelTree/depth=1000/Root=Background-8         383792        367808        -4.16%
BenchmarkContextCancelTree/depth=1000/Root=OpenCanceler-8       384176        368176        -4.16%
BenchmarkContextCancelTree/depth=1000/Root=ClosedCanceler-8     176176        176176        +0.00%

Change-Id: I699ad704d9f7b461214e1651d24941927315b525
Reviewed-on: https://go-review.googlesource.com/25270
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>