Silence nearly matching warning for SortedArray.filter

Swift 4.2 warns that the SortedArray returning version of filter nearly
matches the one required by Sequence. The warning also helpfully tells
us to move it to another extension.

Author: klaaspieter

Sources/SortedArray.swift

@@ -99,7 +99,9 @@ extension SortedArray: RandomAccessCollection {
     public subscript(position: Index) -> Element {
         return _elements[position]
     }
+}
 
+extension SortedArray {
     /// Like `Sequence.filter(_:)`, but returns a `SortedArray` instead of an `Array`.
     /// We can do this efficiently because filtering doesn't change the sort order.
     public func filter(_ isIncluded: (Element) throws -> Bool) rethrows -> SortedArray<Element> {