Add empty and isEmpty to IReadOnlyCollection

Author: gusty

src/FSharpPlus/Extensions/IReadOnlyCollection.fs

@@ -5,8 +5,11 @@ namespace FSharpPlus
 module IReadOnlyCollection =
     open System.Collections.Generic
 
+    [<GeneralizableValue>]
+    let empty<'T> = [||]                                  :> IReadOnlyCollection<'T>
     let ofArray (source: 'T[]   ) = source                :> IReadOnlyCollection<'T>
     let ofList  (source: 'T list) = source                :> IReadOnlyCollection<'T>
     let ofSeq   (source: seq<'T>) = source |> Array.ofSeq :> IReadOnlyCollection<'T>
     let map  mapping (source: IReadOnlyCollection<'T>) = Seq.map  mapping source |> Seq.toArray :> IReadOnlyCollection<'U>
-    let iter mapping (source: IReadOnlyCollection<'T>) = Seq.iter mapping source
+    let iter mapping (source: IReadOnlyCollection<'T>) = Seq.iter mapping source
+    let isEmpty (source: IReadOnlyCollection<'T>) = source.Count = 0