Conditionally conform optional to selection (#204)

* Conditionally conform optional to selection

* wip

Author: stephencelis

Sources/StructuredQueriesCore/Never.swift

@@ -25,3 +25,5 @@ extension Never: Table {
 
   private struct NotDecodable: Error {}
 }
+
+extension Never: _Selection {}

Sources/StructuredQueriesCore/Optional.swift

@@ -183,6 +183,8 @@ extension Optional: Table, PartialSelectStatement, Statement where Wrapped: Tabl
   public typealias Selection = Wrapped.Selection?
 }
 
+extension Optional: _Selection where Wrapped: _Selection {}
+
 extension Optional: PrimaryKeyedTable where Wrapped: PrimaryKeyedTable {
   public typealias Draft = Wrapped.Draft?
 }

Sources/StructuredQueriesCore/TableAlias.swift

@@ -183,6 +183,8 @@ public struct TableAlias<
   }
 }
 
+extension TableAlias: _Selection where Base: _Selection {}
+
 extension TableAlias: PrimaryKeyedTable where Base: PrimaryKeyedTable {
   public typealias Draft = TableAlias<Base.Draft, Name>
 }