Eio.Path: always use "/" as separator

path.mli says:

> In Eio, the directory separator is always "/", even on Windows.

However, `Path.(/)` used `Filename.concat` to create paths, which uses
the native separator ("\" on Windows).

lib_eio/path.ml

@@ -1,11 +1,22 @@
 type 'a t = 'a Fs.dir * Fs.path
 
+(* Like [Filename.is_relative] but always using "/" as the separator. *)
+let is_relative = function
+  | "" -> true
+  | x -> x.[0] <> '/'
+
+(* Like [Filename.concat] but always using "/" as the separator. *)
+let concat a b =
+  let l = String.length a in
+  if l = 0 || a.[l - 1] = '/' then a ^ b
+  else a ^ "/" ^ b
+
 let ( / ) (dir, p1) p2 =
   match p1, p2 with
-  | p1, "" -> (dir, Filename.concat p1 p2)
-  | _, p2 when not (Filename.is_relative p2) -> (dir, p2)
+  | p1, "" -> (dir, concat p1 p2)
+  | _, p2 when not (is_relative p2) -> (dir, p2)
   | ".", p2 -> (dir, p2)
-  | p1, p2 -> (dir, Filename.concat p1 p2)
+  | p1, p2 -> (dir, concat p1 p2)
 
 let pp f (Resource.T (t, ops), p) =
   let module X = (val (Resource.get ops Fs.Pi.Dir)) in

tests/fs.md

@@ -985,3 +985,29 @@ Exception: Failure "Simulated error".
 +seek from end: 9
 - : unit = ()
 ```
+
+# Extending paths
+
+```ocaml
+# run @@ fun env ->
+  let base = fst env#cwd in
+  List.iter (fun (a, b) -> traceln "%S / %S = %S" a b (snd ((base, a) / b))) [
+    "foo", "bar";
+    "foo/", "bar";
+    "foo", "/bar";
+    "foo", "";
+    "foo/", "";
+    "", "";
+    "", "bar";
+    "/", "";
+  ]
++"foo" / "bar" = "foo/bar"
++"foo/" / "bar" = "foo/bar"
++"foo" / "/bar" = "/bar"
++"foo" / "" = "foo/"
++"foo/" / "" = "foo/"
++"" / "" = ""
++"" / "bar" = "bar"
++"/" / "" = "/"
+- : unit = ()
+```