chore(reducers): reduce object copying in iterators

[kuhe]

aws/aws-sdk-js-v3#4151

• reduce object copying in iterators

This reducer style makes pointless object copies via { ...acc}:

.reduce((acc, [key, val]) => ({ ...acc, [key]: val }), {});

We know that removing these copies will have no side-effects because the initial accumulator value is {}, and not an external object that should not be mutated.

New pattern:

.reduce((acc, [key, val]) => (acc[key] = val, acc), {});