[OCaml] Some exception improvements

The OCaml module's exception handling code was poorly designed,
gave confusing exception messages, and was vulnerable to buffer
overflows.

The OCaml module's SWIG_exception_() was adding a useless newline to
the end of the exception message.

In some cases, the integer value of f.e. SWIG_TypeError was being added
to the exception message.

The unneeded else in the OCaml module's SWIG_contract_assert() macro
was causing -Wmisleading-indentation warnings.

The OCaml module's exception handling code now mirrors that of the
Java module.

Add Lib/ocaml/std_except.i.
Add multiple runtime tests.

Examples/test-suite/ocaml/swig_exception_runme.ml

@@ -0,0 +1,13 @@
+open Swig
+open Swig_exception
+
+let _ =
+  let c = new_Circle '(10) and s = new_Square '(10) in
+  assert (_Shape_nshapes '() as int = 2);
+  try
+    ignore (c -> throwException ()); assert false
+  with Sys_error msg -> assert (msg = "OK");
+  let _ = c -> "~" () in
+  let _ = s -> "~" () in
+  assert (_Shape_nshapes '() as int = 0);
+;;