Use latest FSharpSet implementation

Author: alfonsogarciacaro

src/fable-library/Map.fs

@@ -669,7 +669,8 @@ type Map<[<EqualityConditionalOn>]'Key, [<EqualityConditionalOn; ComparisonCondi
                 let m1 = e1.MoveNext()
                 let m2 = e2.MoveNext()
                 (m1 = m2) && (not m1 ||
-                                 (let e1c, e2c = e1.Current, e2.Current
+                                 (let e1c = e1.Current
+                                  let e2c = e2.Current
                                   ((e1c.Key = e2c.Key) && (Unchecked.equals e1c.Value e2c.Value) && loop())))
             loop()
         | _ -> false
@@ -829,19 +830,19 @@ let fold<'Key, 'T, 'State when 'Key : comparison> folder (state:'State) (table:
 
 // [<CompiledName("FoldBack")>]
 let foldBack<'Key, 'T, 'State  when 'Key : comparison> folder (table: Map<'Key, 'T>) (state:'State) =
-    MapTree.foldBack  folder table.Tree state
+    MapTree.foldBack folder table.Tree state
 
 // [<CompiledName("ToSeq")>]
 let toSeq (table: Map<_, _>) =
     table |> Seq.map (fun kvp -> kvp.Key, kvp.Value)
 
 // [<CompiledName("FindKey")>]
 let findKey predicate (table : Map<_, _>) =
-    table |> toSeq |> Seq.pick (fun (k, v) -> if predicate k v then Some k else None)
+    table |> Seq.pick (fun kvp -> let k = kvp.Key in if predicate k kvp.Value then Some k else None)
 
 // [<CompiledName("TryFindKey")>]
 let tryFindKey predicate (table : Map<_, _>) =
-    table |> toSeq |> Seq.tryPick (fun (k, v) -> if predicate k v then Some k else None)
+    table |> Seq.tryPick (fun kvp -> let k = kvp.Key in if predicate k kvp.Value then Some k else None)
 
 // [<CompiledName("OfList")>]
 let ofList (elements: ('Key * 'Value) list) =