Applies the given transformer to all keys in the given record's entries and returns a new record containing the transformed entries.

If the transformed entries contain the same key multiple times, only the last one will appear in the returned record.

Example:

import { mapKeys } from "https://deno.land/std@0.131.0/collections/mod.ts";
import { assertEquals } from "https://deno.land/std@0.131.0/testing/asserts.ts";

const counts = { a: 5, b: 3, c: 8 }

assertEquals(mapKeys(counts, it => it.toUpperCase()), {
    A: 5,
    B: 3,
    C: 8,
})