Replace Bond with IRB

[cfis]

This pull request replaces bond with IRB. Bond is a gem that hasn't been updated in 6 years, requires a native extension, and does not compile with MSVC. Instead, the PlainBackend is switched over to IRB which is installed with Ruby.

[kojix2]

Hi @cfis Thank you for your Pull request. It would be great if we could replace bond with IRB. I'll show it to someone familiar with the IRB... So please wait.

[cfis]

Thoughts?

[kojix2]

I agree with dropping bond. But there may be some issues.

1. in case of an error, Backend class objects should return eval_path.

def eval_path
  @irb.context.irb_path
end

The current pull request causes problems here.

https://github.com/SciRuby/iruby/blob/f494ef1b048b26dadea614ea360fc06a999363e2/lib/iruby/kernel.rb#L114

2. IRB execute code at Binding in the Object::TOPLEVEL_BINDING function. This causes the history module to fail. But I think the original approach may not be so appropriate...

3. Are these codes necessary? I don't know well...

IRB.conf[:MAIN_CONTEXT] = @irb.context

# Called by irb
def set_encoding(extern, intern)
  a = extern
end

ping @mrkn, what do you think?

[mrkn]

I cannot continue to work this pull request because this repository was detached from minrk/iruby. I'll continue to work this in the new pull request.

[mrkn]

I fixed this by #297.