events: improve listeners() performance

[mscdex]

Benchmark results (keep in mind listeners is always implicitly multiplied by 2 by the benchmark):

 confidence improvement accuracy (*) (**) (***) events/ee-listeners.js raw='false' listeners=1 n=5000000 -0.96 % ±2.89% ±3.85% ±5.03% events/ee-listeners.js raw='false' listeners=10 n=5000000 *** 7.71 % ±1.48% ±1.98% ±2.61% events/ee-listeners.js raw='false' listeners=100 n=5000000 *** 35.04 % ±0.89% ±1.19% ±1.58% events/ee-listeners.js raw='false' listeners=2 n=5000000 *** 20.62 % ±1.41% ±1.89% ±2.47% events/ee-listeners.js raw='false' listeners=5 n=5000000 *** 27.57 % ±0.80% ±1.07% ±1.40% 

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/31853/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/31893/

[jasnell]

Landed in a4f3206