Extract Fiber::StackPool

[ysbaddaden]

Keeps stack operations (allocate, recycle, free) out of Fiber itself.

Just a mere extraction; not MT-safe but will be easy to add —e.g. use a mutex to protect the deque.pop?, deque.push and deque.shift? calls.

[ysbaddaden]

The circle/linux job failed, but the error seems unrelated:

1) TCPSocket fails when connection is refused
   Failure/Error: expect_raises_errno(Errno::ECONNREFUSED, "Error connecting to 'localhost:#{port}'") do
     Expected Errno but nothing was raised
   # spec/std/socket/tcp_socket_spec.cr:132

[asterite]

We should note those CI errors and try to fix them. Lately I've been trying to get a CI that we can trust but it seems we are still missing a few cases.

[straight-shoota]

@asterite The hard thing is recognizing repeated instances of the same CI failure. A single random connection error simply isn't really worth noting because it could happen for many reasons and most of them are out of our control.

[sdogruyol]

Are we holding this just because of the CI failure?

[asterite]

I just re-ran it.

[sdogruyol]

@asterite should be good to 🚢

[asterite]

Looks good! Just a small request

[straight-shoota]

Should be ready to merge?