libsmartcols: Export internally used types to API

libsmartcols: Export internally used types to API

We anticipate a need for improved interaction between applications and
the library in certain scenarios. For instance, when a table contains
strings that cannot be readily converted from strings to numbers
(e.g., strings like "15.3MiB") but the application possesses data in a
more suitable format.

Signed-off-by: Karel Zak <kzak@redhat.com>