Printexc: OCaml-friendly access to individual backtrace slots

(Patch by Jacques-Henri Jourdan)

There are several changes:

- `raw_backtrace` is no longer an abstract type, but rather an
  `raw_backtrace_slot array`, where `raw_backtrace_slot` is a new
  abstract type. `raw_backtrace_slot` elements are hashable and
  comparable. At runtime, values of this type contain either
  a bytecode pointer or a frame_descr pointer. In order to prevent the
  GC from walking through this pointer, the low-order bit is set to
  1 when stored in the array.

- The old `loc_info` type is know public, renamed into `backtrace_slot`:

      type backtrace_slot =
        | Known_location of bool   (* is_raise *)
                          * string (* filename *)
                          * int    (* line number *)
                          * int    (* start char *)
                          * int    (* end char *)
        | Unknown_location of bool (*is_raise*)

- new primitive :

    val convert_raw_backtrace_slot: raw_backtrace_slot -> backtrace_slot

  Rather than returning an option, it raises Failure when it is not
  possible to get the debugging information. It seems more idiomatic,
  especially because the exceptional case cannot appear only for a part
  of the executable.

- the caml_convert_raw_backtrace primitive is removed; it is more
  difficult to implement in the C side because of the new exception
  interface described above.

- In the bytecode runtime, the events are no longer deserialized once
  for each conversion, but once and for all at the first conversion,
  and stored in a global array (*outside* the OCaml heap), sorted by
  program counter value. I believe this information should not take
  much memory in practice (it uses the same order of magnitude memory
  as the bytecode executable). It also makes location lookup much more
  efficient, as a dichomoty is used instead of linear search as
  previously.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14776 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
typos

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14777 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
cleanup cmp_ev_info

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14778 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
factor the pointers-into-ints cleverness through pair of macros

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14779 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
reinstate the deprecated primitive caml_get_exception_backtrace

Jacques-Henri initially removed the primitive, which is deprecated
since 4.01, but I suspect there still are uses in the wild. I guess we
should wait for a few more versions.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14780 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
expose Printexc.format_backtrace_slot to facilitate testing

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14781 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
improve backtrace testsuite

Test the behavior of the deprecated primitive [caml_get_exception_backtrace],
and minimal tests for hashing/comparison of raw backtrace slots.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14782 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
bootstrap

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14783 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Reformulation of the user-facing slot-access API

- The internal [backtrace_slot] type is not exposed anymore, instead
  accessors function return orthogonal information
  (is_raise, location). This is both more extensible and more
  user-friendly.

- The [raw_backtrace_slot] is exposed separately as a low-level type
  that most users should never use. The unsafety of marshalling is
  documented. Instead of defining
  [raw_backtrace = raw_backtrace_slot array], I kept [raw_backtrace]
  an abstract type with [length] and [get] functions for
  random-access. This should allow us to change the implementation in
  the future to be more robust wrt. marshalling (boxing the trace in
  a Custom block, or even possibly the raw slots at access time).

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14784 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02