Replace `create` with `to_timeline` or some such?

[light-matters]

It is still a point of contention that create and update are virtually identical apart from their ability to stack. I'm not sure we can really get around this in terms of number of functions, but it might make sense to have some sort of 'timeline.resolve' function to' force update(...) chains to resolve to a timeline instead of a function. This would give the functions two clear, distinct, purposes rather than leaving the user confused over when to use create and when to use update.

It would also address #45 , which I am reluctant to affect otherwise.