encapsulate unsafe list-accumulators in an abstract module

This patch is still incomplete, in particular it lacks tests for most
of the functions changed, but it demonstrates the possibility to
capture all the accumulator usage under a common abstraction. The code
is provably safe (equivalent to another imperative implementation with
(type 'a mut_list = 'a list ref) and no Obj.magic at all).

The only function that gave me trouble is 'transpose', as it creates
an unbounded number of intermediate accumulators. The resulting
implementation, using continuation-passing to not break
tail-recursivity, is admittedly much less readable than the previous
one. Is it a fair cost to pay for safety by encapsulation?

Makefile

@@ -13,7 +13,7 @@ export DOCROOT
 BROWSER_COMMAND ?= x-www-browser
 export BROWSER_COMMAND
 
-OCAMLBUILD ?= ocamlbuild
+OCAMLBUILD ?= ocamlbuild -use-ocamlfind
 OCAMLBUILDFLAGS ?= -no-links
 
 ifeq ($(uname_S),Darwin)
@@ -193,9 +193,9 @@ test-native: prefilter _build/testsuite/main.native _build/$(QTESTDIR)/all_tests
 full-test: $(TEST_TARGET)
 
 test-compat: prefilter src/batteries_compattest.ml
-	ocamlbuild src/batteries_compattest.byte -no-links
+	ocamlbuild -use-ocamlfind src/batteries_compattest.byte -no-links
 
-test: test-byte test-compat
+test: test-native test-compat
 
 ###############################################################################
 #	BENCHMARK SUITE