Support seamless mutation promise resolution

[drborges]

It would be very convenient if mutations could take promises in addition to sync values, example:

store.users.push(fetch("app.com/users/123"))
store.todos = fetch("app.com/todos?limit=20")

If the mutation value is a Promise, arbor would then only apply the corresponding mutation upon promise resolution.

Failures can still be handled by the caller:

const userPromise = fetch("app.com/users/123").catch(console.log)
store.users.push(userPromise)

This is particularly useful when APIs reflect exactly the data structure expected by the front-end (BFF pattern).