User-defined aggregate functions

[stephencelis]

We introduced the @DatabaseFunction macro back in #151, which allowed for user-defined scalar database functions. SQLite also offers the ability to define aggregate functions, so this PR augments the macro to do just that.

If a function is annotated with @DatabaseFunction and takes some Sequence of database-decodable values, it will be installed as an aggregate.

As a silly example, we could rewrite the sum function in Swift:

@DatabaseFunction
func sum(_ ints: some Sequence<Int>) -> Int {
  ints.reduce(0, +)
}

Reminder.select { $sum($0.id) }
// SELECT "sum"("reminders"."id")
// FROM "reminders"
// => 55

[stephencelis]

With this public, should we rename AggregateFunction to AggregateFunctionExpression?

Should we also rename QueryFunction to FunctionExpression (or ScalarFunctionExpression) and publicize its initializer?