Speed-up file access by not using lseek

This is related to performance improvments mentionned in ocaml#412

src/core/opamFile.ml

@@ -1442,23 +1442,8 @@ module Make (F : F) = struct
 
   let dummy_file = OpamFilename.of_string "<dummy>"
 
-  let string_of_channel ic =
-    let n = 32768 in
-    let s = String.create n in
-    let b = Buffer.create 1024 in
-    let rec iter ic b s =
-      let nread =
-        try input ic s 0 n
-        with End_of_file -> 0 in
-      if nread > 0 then (
-        Buffer.add_substring b s 0 nread;
-        iter ic b s
-      ) in
-    iter ic b s;
-    Buffer.contents b
-
   let read_from_channel ic =
-    try F.of_string dummy_file (string_of_channel ic)
+    try F.of_string dummy_file (OpamSystem.string_of_channel ic)
     with OpamFormat.Bad_format msg ->
       OpamGlobals.error_and_exit "%s" msg
 

src/core/opamSystem.ml

@@ -69,11 +69,25 @@ let remove_file file =
   try Unix.unlink file
   with Unix.Unix_error _ -> ()
 
+
+let string_of_channel ic =
+  let n = 32768 in
+  let s = String.create n in
+  let b = Buffer.create 1024 in
+  let rec iter ic b s =
+    let nread =
+      try input ic s 0 n
+      with End_of_file -> 0 in
+    if nread > 0 then (
+      Buffer.add_substring b s 0 nread;
+      iter ic b s
+    ) in
+  iter ic b s;
+  Buffer.contents b
+
 let read file =
   let ic = open_in_bin file in
-  let n = in_channel_length ic in
-  let s = String.create n in
-  really_input ic s 0 n;
+  let s = string_of_channel ic in
   close_in ic;
   s
 

src/core/opamSystem.mli

@@ -42,6 +42,9 @@ val link: string -> string -> unit
     expanded and relative paths become absolute. *)
 val real_path: string -> string
 
+(** Return the contents of a channel. *)
+val string_of_channel: in_channel -> string
+
 (** [read filename] returns the contents of [filename] *)
 val read: string -> string