Wip sip23 √

I agree that `prepare` isn't a great API as-is for the reasons highlighted, but I do believe it's repairable. I seem to have misunderstood your intent here, that you needn't rely on an `implicit def` to make this work.

However, this severely compromises modularity: capture is a property of the EC, not of its uses (e.g. a Promise impl.) If I'm using Twitter's EC, I expect my capture to happen everywhere. Otherwise, for example, I can't reliably do things like propagate locals through third-party libraries.