fiber: basic api exports

Closes #9237

Add exports for fiber_set_name_n, fiber_name, fiber_id, fiber_csw &
fiber_find.

Also make fiber_set_joinable, fiber_set_ctx & fiber_get_ctx interpret
NULL as the current fiber.

@TarantoolBot document
Title: add basic fiber api to ffi exports.

5 basic functions can now be used via ffi api, which were previously
only accessible via lua api: fiber_set_name_n, fiber_name, fiber_id,
fiber_csw & fiber_find.

fiber_set_joinable now interprets NULL as current fiber.