Optionally handle merging maps/sets

[goliatone]

Same as with #7 being able to handle arrays, we should be able to merge maps/sets.
Note that Object.assign would you overwrite the property.

[goliatone]

let source = new Set(...);
let target = new Set(...);

source.forEach(target.add, source);

let source = new Map(...);
let target = new Map(...);

source.forEach((value, key) => target.set(key, value));