Turn remaining value_contents functions into methods

This turns the remaining value_contents functions -- value_contents,
value_contents_all, value_contents_for_printing, and
value_contents_for_printing_const -- into methods of value.  It also
converts the static functions require_not_optimized_out and
require_available to be private methods.

Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/python/py-inferior.c

@@ -714,7 +714,7 @@ infpy_thread_from_thread_handle (PyObject *self, PyObject *args, PyObject *kw)
   else if (gdbpy_is_value_object (handle_obj))
     {
       struct value *val = value_object_to_value (handle_obj);
-      bytes = value_contents_all (val).data ();
+      bytes = val->contents_all ().data ();
       bytes_len = val->type ()->length ();
     }
   else