Issue Description:

Currently FlattenSequence does not conditionally conform to LazySequenceProtocol when its base does, however, this behavior is supported through this implementation in the standard library:

extension LazySequenceProtocol where Element: Sequence {
  /// Returns a lazy sequence that concatenates the elements of this sequence of
  /// sequences.
  @inlinable // lazy-performance
  public __consuming func joined() -> LazySequence<FlattenSequence<Elements>> {
    return FlattenSequence(_base: elements).lazy
  }
}

This is an uncommon and convoluted way to add lazy behavior to FlattenSequence (when base does), the classic approach in the standard library is the following one:

extension Slice: LazySequenceProtocol where Base: LazySequenceProtocol { }
extension ReversedCollection: LazySequenceProtocol where Base: LazySequenceProtocol { }

This minor issue may feel awkward because the returning type of someSequenceOfSequence.lazy.joined() is not FlattenSequence<LazySequence<...>> as expected but LazySequence<FlattenSequence<...>>.
Fixing this would reduce the number of overloads of the joined() method to 1 and make the standard library more consistent.