Fix C++/OCaml wrappers.

Fix C++/OCaml wrappers.
- sxsi_alloc_custom<X>() Allocate an OCaml_custom block holding a
	  		 C++ value of type X
			 Values of type X where X is a pointer type
			 *must* be allocated with new. delete will be
			 called when the custom_block is finalized by
			 the garbage collector

- Obj_val<X>(value v)    Extract a C++ value of type X from a custom
	  		 Block v

git-svn-id: svn+ssh://idea.nguyen.vg/svn/sxsi/trunk/xpathcomp@1181 3cdefd35-fc62-479d-8e8d-bae585ffb9ca