Some background for this refactor:
1. Through a RubyLex instance's lifetime, the context passed to its methods
should be the same.
Given that `Context` is only initialised in `Irb#initialize`,
this should be true.
2. When `RubyLex` is initialised, the context object should be accessible.
This is also true in all 3 of `RubyLex.new`'s invocations.
With the above observations, we should be able to store the context in `RubyLex`
as an instance variable. And doing so will make `RubyLex`'s instance methods
easier to use and maintain.
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