(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
The command prompt on Windows always uses Unicode to take input and print
output but most Reline implementation depends on Encoding.default_external.
This commit introduces an abstracted structure about the encoding of Reline.
