Use contrib dir to do self-contained build with custom OCaml toolchain. Still BROKEN until I update to use new Lem ELF model.

--HG--
rename : src/ocaml/Makefile => src/Makefile-ocaml
rename : src/ocaml/loading.ml => src/loading.ml
rename : src/ocaml/externals.c => src/ocaml-externals.c