perf(lib): replace accumulator spread with object.assign

[Fdawgs]

Ended up stumbling across this blog post over the holidays, which covers why using spread ({...obj}) syntax in accumulators like reduce is a bad idea.

TL;DR: it causes a time complexity of O(n^2) instead of O(n) because it's creating an increasingly growing new object on every iteration. With object.assign, we're mutating the accumulator rather than creating a new object.

Checklist

• run npm run test and npm run benchmark
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[gurgunday]

Nice!