Simplify internals by building useTrail atop useSprings

[aleclarson]

@drcmda noticed that all of the useTrail-specific logic could be removed since useTrail is really just a more constrained useSprings.

Take this for example:

// Shorthand form (more constrained)
const props = useTrail(10, { x: 0, from: { x: 100 } })

// Identical behavior with useSprings
const props = useSprings(10, i => ({ x: 0, from: { x: 100 }, config: { mass: 1 + i } }))

// But useSprings is more flexible (because each spring has its own props)
const props = useSprings(10, i => ({ x: 0, y: i * 10, from: { x: 100, y: 0 }, config: { mass: 1 + i } }))

[aleclarson]

The attach and parent props actually come in handy even without useTrail, so we'll be keeping them. :)

[aleclarson]

Note: In #808, the attach and parent props are replaced by to accepting FluidValue types.

[drcmda]

just was talking about this with @dbismut today, so it appears like it's not exactly the same, see: https://codesandbox.io/s/react-use-gesture-template-hvscw

so this would be hard to reproduce, the closest i've gotten is: https://codesandbox.io/s/react-use-gesture-template-zvgig falls apart when mouse holds still, this is what the internal hacks make possible, that springs somehow can keep on going if they trail.

but either way, i still think useSprings has lots of advantages over trail since it's flexible and often looks more natural. i mean, unless people are really making a snake game adding up mass is probably better overall and saves us a ton of trouble with the internals.

what say you?

[dbismut]

I say we should keep it as a commodity hook at least, but it would be great to be able to select which index should be used as the "trailer" index (#644)

[aleclarson]

Update: useSprings cannot easily achieve the same effect as useTrail, so we're keeping it. See its #808 implementation here.