Improve performance of String.concat for seq of type array or list (#9483)

* Move `String.length` to the top of its module so that the `length` function is in scope

* Improve performance of String.concat for arrays and lists

* Make concatArray local to String.concat

* Testing String.concat, make sure the new three paths are covered

* Remove "foo", "bax", "bar" in tests, making them more readable

Co-authored-by: Phillip Carter <pcarter@fastmail.com>

Author: 2 people

src/fsharp/FSharp.Core/string.fs

@@ -8,6 +8,7 @@ namespace Microsoft.FSharp.Core
     open Microsoft.FSharp.Core.Operators
     open Microsoft.FSharp.Core.Operators.Checked
     open Microsoft.FSharp.Collections
+    open Microsoft.FSharp.Primitives.Basics
 
     [<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
     [<RequireQualifiedAccess>]
@@ -22,7 +23,25 @@ namespace Microsoft.FSharp.Core
 
         [<CompiledName("Concat")>]
         let concat sep (strings : seq<string>) =  
-            String.Join(sep, strings)
+
+            let concatArray sep (strings: string []) =
+                match length sep with
+                | 0 -> String.Concat strings
+                // following line should be used when this overload becomes part of .NET Standard (it's only in .NET Core)
+                //| 1 -> String.Join(sep.[0], strings, 0, strings.Length)
+                | _ -> String.Join(sep, strings, 0, strings.Length)
+
+            match strings with
+            | :? array<string> as arr -> 
+                concatArray sep arr
+
+            | :? list<string> as lst -> 
+                lst 
+                |> List.toArray 
+                |> concatArray sep
+
+            | _ ->
+                String.Join(sep, strings)
 
         [<CompiledName("Iterate")>]
         let iter (action : (char -> unit)) (str:string) =