Add an `alter/convert` function to modify a snowball

[RayBenefield]

A common pattern I've noticed is:

transmute({ roar: 'test' })
	.extend('afterChange', snowball =>
		func({ ...snowball, differentName: snowball.roar })
	)

It should be something more like:

transmute({ roar: 'test' })
	.extend('afterChange',
		alter('roar', 'differentName')(func)
    )

transmute({ roar: 'test', dude: 'stuff' })
	.extend('afterChange',
		alter(
			['roar', 'dude'],
			['differentName', 'betterName'],
			func
		)
    )

Maybe... might be powerful. Not really a fan of the syntax yet, but would be interesting. Alternatively a solid thing would be a babel plugin that allows this:

transmute({ roar: 'test' })
	.extend('afterChange', (snowball: { roar: differentName }) =>
		func({ ...snowball, differentName })
	)

Destructuring on positional arguments. Just some thoughts.