Use RMutex for locks (references #46).

There seem to be some lock re-entrancy bugs in `BatIO` that cause
hangs when we use a plain mutex for synchronization.  Using RMutex
until those problems are fixed or we determine that they are inherent.

src/batteries.ml

@@ -2,7 +2,7 @@ include Batteries_uni (* the meat of this module is in batteries_uni.mli *)
 module Mutex = BatMutex;;
 module RMutex = BatRMutex;;
 
-Unix.lock := Mutex.Mutex.make ();;
-IO.lock := Mutex.Mutex.make ();;
-IO.lock_factory := Mutex.Mutex.make;;
-lock := Mutex.Mutex.make ();;
+Unix.lock := RMutex.make ();;
+IO.lock := RMutex.make ();;
+IO.lock_factory := RMutex.make;;
+lock := RMutex.make ();;