improve then sequences

[stefanpenner]

https://github.com/stefanpenner/promise_benchmarks/blob/master/lib/tests/micro/sequence.js

Running Benchmark Suite for test - Then sequence 1 ...
- [Then sequence 1] es6 x 939,479 ops/sec ±18.17% (55 runs sampled)
- [Then sequence 1] es6Promise x 221,425 ops/sec ±17.20% (51 runs sampled)
- [Then sequence 1] bluebird x 1,062,344 ops/sec ±17.98% (60 runs sampled)
- [Then sequence 1] when x 1,368,123 ops/sec ±20.26% (57 runs sampled)
Running Benchmark Suite for test - Then sequence 2 ...
- [Then sequence 2] es6 x 647,827 ops/sec ±15.82% (54 runs sampled)
- [Then sequence 2] es6Promise x 157,352 ops/sec ±20.13% (40 runs sampled)
- [Then sequence 2] bluebird x 908,095 ops/sec ±16.80% (62 runs sampled)
- [Then sequence 2] when x 1,235,377 ops/sec ±14.46% (62 runs sampled)
Running Benchmark Suite for test - Then sequence 10 ...
- [Then sequence 10] es6 x 208,993 ops/sec ±21.62% (53 runs sampled)
- [Then sequence 10] es6Promise x 50,187 ops/sec ±21.17% (42 runs sampled)
- [Then sequence 10] bluebird x 334,191 ops/sec ±18.08% (60 runs sampled)
- [Then sequence 10] when x 890,853 ops/sec ±15.25% (66 runs sampled)
Running Benchmark Suite for test - Then sequence 50 ...
- [Then sequence 50] es6 x 50,806 ops/sec ±17.34% (54 runs sampled)
- [Then sequence 50] es6Promise x 13,661 ops/sec ±17.86% (36 runs sampled)
- [Then sequence 50] bluebird x 84,802 ops/sec ±14.24% (58 runs sampled)
- [Then sequence 50] when x 337,793 ops/sec ±10.03% (63 runs sampled)
Running Benchmark Suite for test - Then sequence 100 ...
- [Then sequence 100] es6 x 27,147 ops/sec ±11.96% (54 runs sampled)
- [Then sequence 100] es6Promise x 7,078 ops/sec ±12.77% (34 runs sampled)
- [Then sequence 100] bluebird x 43,338 ops/sec ±11.36% (58 runs sampled)
- [Then sequence 100] when x 200,033 ops/sec ±8.15% (63 runs sampled)
Running Benchmark Suite for test - Then sequence 1000 ...
- [Then sequence 1000] es6 x 2,651 ops/sec ±11.70% (55 runs sampled)
- [Then sequence 1000] es6Promise x 843 ops/sec ±11.40% (48 runs sampled)
- [Then sequence 1000] bluebird x 4,851 ops/sec ±9.46% (64 runs sampled)
Fastest for Then sequence 1000 is [ 'lie' ]
Running Benchmark Suite for test - Then sequence 10000 ...
- [Then sequence 10000] es6 x 232 ops/sec ±17.72% (50 runs sampled)
- [Then sequence 10000] es6Promise x 74.30 ops/sec ±16.88% (51 runs sampled)
- [Then sequence 10000] bluebird x 443 ops/sec ±16.93% (64 runs sampled)
- [Then sequence 10000] when x 2,115 ops/sec ±9.78% (62 runs sampled)