Applies the given transformer to all valuesin the given record and returns a new record containing the resulting keys associated to the last value that produced them.

Example:

const usersById = {
    'a5ec': { name: 'Mischa' },
    'de4f': { name: 'Kim' },
}
const namesById = mapValues(usersById, it => it.name)

console.assert(namesById === {
    'a5ec': 'Mischa',
    'de4f': 'Kim',
}