Add --context-mode 5: evaluates inside Ruby::Box

[tompng]

A playground to try monkey patching core methods.

% RUBY_BOX=1 irb --context-mode 5          
ruby: warning: Ruby::Box is experimental, and the behavior may change in the future!
See doc/box.md for known issues, etc.
Context-mode-5 (binding in new Ruby::Box) is experimental. It may be removed or changed without notice.
irb(main):001> defined? IRB
=> nil
irb(main):002* class Integer
irb(main):003*   def +(*) = 42
irb(main):004> end
=> :+
irb(main):005> 1 + 2
=> 42
irb(main):006> 

Related to: #143 #235 #960

[ima1zumi]

How about we include a check for the Ruby version?

[st0012]

I think it should be a constant defined check instead? If Ruby::Box doesn't exist, then we should warn that it's not available and that we'd fallback to the default behaviour.

[tompng]

Add a defined?(Ruby::Box) check.
I think if Ruby::Box is unavailable, it's OK or even safe to fail instead of fallback to default.

There is one more error case. If Box is not enabled by ENV['RUBY_BOX'], class Ruby::Box is defined but calling new throws RuntimeError.

'Ruby::Box#initialize': Ruby Box is disabled. Set RUBY_BOX=1 environment variable to use Ruby::Box. (RuntimeError)

[st0012]

Can we have a few simple test cases running with this mode on? It can be used to detect Ruby::Box's regression and we can also proactively disable it if it becomes noisy.

[tompng]

Immediate exit will make IRB.start rescue do_something not work as expected