ocamlPackages.ocaml_extlib-1-7-7: init at 1.7.7

Unfortunately there's no way to get Haxe 4.0 and 4.1 to work with extlib
1.7.8 (not even without the minimal install), so we need to package
1.7.7 again, at least until 1.7.9 (?) brings backwards compatibility
packages, hopefully.

pkgs/development/ocaml-modules/extlib/1.7.7.nix

···
+# Older version of extlib for Haxe 4.0 and 4.1.
+# May be replaceable by the next extlib + extlib-base64 release.
+{ fetchurl, ocaml_extlib }:
+
+ocaml_extlib.overrideAttrs (_: rec {
+  version = "1.7.7";
+  src = fetchurl {
+    url = "https://github.com/ygrek/ocaml-extlib/releases/download/${version}/extlib-${version}.tar.gz";
+    sha256 = "1sxmzc1mx3kg62j8kbk0dxkx8mkf1rn70h542cjzrziflznap0s1";
+  };
+})

pkgs/top-level/ocaml-packages.nix

···
 
     ocaml-protoc = callPackage ../development/ocaml-modules/ocaml-protoc { };
 
-    ocaml_extlib = callPackage ../development/ocaml-modules/extlib { };
+    ocaml_extlib = ocaml_extlib-1-7-8;
+
+    ocaml_extlib-1-7-8 = callPackage ../development/ocaml-modules/extlib { };
+
+    ocaml_extlib-1-7-7 = callPackage ../development/ocaml-modules/extlib/1.7.7.nix { };
 
     ocb-stubblr = callPackage ../development/ocaml-modules/ocb-stubblr { };