Adds Peano.Int63 in ocaml64 driver

     and renames to_int63 into to_int63_partial

drivers/ocaml64.drv

@@ -302,6 +302,12 @@ module mach.peano.MinMax
   syntax val  min    "min %1 %2" prec 4 3 3
 end
 
+module mach.peano.Int63
+  syntax val  defensive_to_int63    "%1"
+  syntax val  to_int63    "%1"
+  syntax val  of_int63    "%1"
+end
+
 module mach.onetime.OneTime
   syntax type t      "int"
   syntax val  to_int "Z.of_int %1" prec 4 3

stdlib/mach/peano.mlw

@@ -102,10 +102,19 @@ module MinMax
 end
 
 module Int63
+  use int.Int
   use mach.int.Int63
   use Peano
 
+  val defensive_to_int63 (x:t) : int63
+    requires { Int63.in_bounds x.v }
+    ensures { result = x.v }
+
   val partial to_int63 (x:t) : int63
     ensures { result = x.v }
 
+  val of_int63 (x:int63) (low high: t) : t
+    requires { low.v <= x <= high.v }
+    ensures { result.v = x }
+
 end