lib: rename functions to clearly indicate API inheritance

This patch renames some functions of the library's API to show how
an API inherits another one, for example (before):

    bt_field_signed_integer_get_value()
    bt_field_unsigned_integer_get_value()

vs. (after):

    bt_field_integer_signed_get_value
    bt_field_integer_unsigned_get_value

The second version clearly shows that both are part of the
`bt_field_integer` API.

This patch aligns the names of those functions with other parts of the
API which already have the correct order of prefixes, for example:

    bt_component_class_source_create()
    bt_component_class_filter_create()
    bt_component_class_sink_create()
    bt_component_class_get_name()
    bt_message_event_create()
    bt_message_get_type()

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I5f7b637d1d72d610f88f3f7d0122c8975f26481b
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1728
Tested-by: jenkins <jenkins@lttng.org>