Bring all() types closer to Promise.all()

[cowboyd]

Motivation

#870

Non-goals

1. It does not need to support non-operation types like primitives, null, or undefined; unlike Promise.all() which can receive non-Promise values.
2. Does not (currently) need to support iterables. This will come later.

Approach

I used the typings for Promise.all() as a basis for the typings for all(). The only thing missing was that we needed an equivalent to the Awaited type, so this includes a Yielded<T> type which can be used to unwrap the type of an Operation. For example:

Yielded<Operation<string>> // => string
Yielded<Operation<number>> // => number

The Yielded type is not exported, but probably should be at some point.