Add a generic get_value() implementation

Use get_value to get a field's value. Currently it does not support
compound types. Also it does not support floating point, since getting
a float's value is not implemented in babeltrace now.

Edit by Jérémie Galarneau: Arrays' value should not be returned using
get_char_array(). Implemented get_array_element_at() which returns a
Definition object corresponding to the array's element.

Exposed the bt_array_index function in the Python bindings

Signed-off-by: Xiaona Han <xiaonahappy13@163.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>