[ruby/irb] Store context in RubyLex

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.

https://github.com/ruby/irb/commit/5c8d3df2df

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