Add helper methods to reduce a sequence into an object or another sequence

[dimitribouniol]

Most of the time, users want to reduce a sequence, of say Bytes, into a single object.

func reduce<T>(into: T.Type = T.self, transformer: (iterator) async throws -> T) async throws -> T {
    var iterator = self.makeAsyncIterator()

    let result = try await transformer(iterator)
    
    /// Make sure we are EOF
    guard try await iterator.next() == nil
    else { throw ... }
    
    return result
}