Motion component variant API doesn't support motion groups

[robertpenner]

• Our createPresenceComponentVariant function assumes that enter and exit are each a single motion atom.
• We need to support motion groups, where enter or exit can be an array of motion atoms.
• Motion groups are essential for even simple choreography like a delayed collapse/expand:
  • enter by first expanding the height
  • Then fade in the content, using delay
  • The above are 2 motion atoms, grouped in a array and assigned to enter.
• A variant of the delayed collapse could have different values of the delay, durations or easings.
• A simpler approach with factory functions is possible.

[robertpenner]

I found a simpler functional approach that makes createPresenceComponentVariant unnecessary.