Add fiber attributes

Fiber attributes are a way to specify parameters that is different
from the default. When a fiber is created using fiber_new_ex(),
an attribute object can be specified to configure custom values for
some options. Attributes are specified only at fiber creation time;
they cannot be altered while the fiber is being used.

Currently only stack_size attribute is supported. Fibers with
non-default stack size won't be recycled via fiber pool.

API overview:

* fiber_new_ex() creates a fiber with custom attributes.
* fiber_attr_new()/fiber_attr_delete() creates/destroys attributes.
* fiber_attr_setstacksize()/fiber_attr_getstacksize() sets/gets
  the fiber stack size.
* fiber_self() returns current running fiber.

All new functions are available from public C API for modules.

See #2438