Fix problem with sequenceA.
Fix problem with sequenceA.

This was introduced in #f80ad7e6a3ed5968f950ef24f044d9e0ced10f65 by adding an overload for seq it 'eats' all subtypes of seq. This was discovered when doing let x = sequenceA [|Some 1; Some 2|] and x returns a seq instead of an array.
gmpl/FSharpPlus,gusty/FSharpPlus,mausch/FsControl,gmpl/FSharpPlus,gmpl/FsControl,gusty/FSharpPlus