improve performance of normalizeList

[Uzlopak]

before:

node benchmarks/normalizeList.js
false x 80,026,396 ops/sec ±1.11% (189 runs sampled)
valid single x 6,009,549 ops/sec ±0.85% (175 runs sampled)
valid array x 56,498,603 ops/sec ±1.91% (175 runs sampled)

after:

node benchmarks/normalizeList.js
false x 1,233,563,481 ops/sec ±0.23% (193 runs sampled)
valid single x 1,230,255,179 ops/sec ±0.24% (194 runs sampled)
valid array x 218,264,955 ops/sec ±0.33% (190 runs sampled)

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm