lib: make values API const-correct

This patch sets out the ground for subsequent const-correctness patches.
The rules are:

* Convert private API to non-const API, without any `private` prefix,
  including in the file name. A non-const API function accept a
  non-const object.

  Put common enumerations in this header.

* Convert public API to const API:

  * Use `-const.h` suffix in file name.

  * Use `_const` suffix to borrowing functions and callback type
    definitions.

In this particular patch, the `bt_value_null` singleton object is
non-const because you can use it with bt_value_array_append_element()
and bt_value_map_insert_entry().

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>