fiber: introduce fiber_o:info() and fiber_o:csw()

If you want to get information or get csw (Context SWitch) of some fiber
you need to call fiber.info(), but it creates table with information about
all the fibers. This patch introduces fiber_object:info() and
fiber_object:csw() - functions to solve this problem.

Closes #5799

@TarantoolBot document
Title: introduce fiber_object:info() and fiber_object:csw()
```
-- fiber_object:info() is the same as fiber.info(), but show information only
about one alive fiber.
-- fiber_object:csw() show csw (Context SWitch) of alive fiber.
```