Make fibers RUBY_TYPED_THREAD_SAFE_FREE

Now fibers can be collected by multiple ractors at once when they are
running local GCs. Fibers are created in the Ractor that owns them, so
if multiple Ractors have unmarked fibers during local GC sweeping, we
can now collect them concurrently and in parallel instead of deferring
their collection to a postponed job.

Author: luke-gruber