Add variadic coalesce

Currently `coalesce` can only take two values as arguments. It's relatively common to need to coalesce multiple values. This involves writing nested `coalesce` calls.

`concat` is a similar accumulator function. It has a variadic variant which is very helpful. It would be helpful to do the same with `coalesce`.

Start by locating the specification's definition of coalesce and the variadic variant of concat. Compare how the two functions describe their arguments, then define the expected variadic coalesce behavior in the specification. Done means the specification clearly supports coalescing multiple values without nested calls.

Written by the indexing model from the issue text.