events: improve listeners() performance

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