(https://github.com/ruby/reline/pull/566)

* Avoid referencing IOGate from IOGate classes

The only time those classes being used is when themselves being the IOGate.
So when referencing to IOGate, it's better to use `self` instead.

* Avoid referencing to IOGate from LineEditor directly

* Avoid referencing to IOGate from Core directly

* Reference to Reline.core directly

* Replace Reline::IOGate with Reline.core.io_gate