Implement groupBy

Write groupBy(array, iteratee): return an object mapping each key to the items that produced it, in input order.

• iteratee is a function, or a property name to read from each item, such as "length" or "type".
• Keys are strings, as object keys always are.
• Every key must work, including "constructor" and "toString", which a plain {} already inherits.

groupBy([6.1, 4.2, 6.3], Math.floor)   // { "4": [4.2], "6": [6.1, 6.3] }
groupBy(["one", "two", "three"], "length")   // { "3": ["one", "two"], "5": ["three"] }

Object.groupBy and Map.groupBy are switched off while your code runs. The functions named in the tests (floor, identity and parity) are supplied by the grader.