Restore IRB.CurrentContext when binding.irb is nested

[tompng]

Fix #650

$ irb
irb(main):001> 123.instance_eval{binding.irb}
irb(123):001> exit
=> nil
irb(main):002> IRB.CurrentContext.main
=> 123 # Should be `main`. `IRB.CurrentContext` is not restored

Changes

Moved preparation of conf[:MAIN_CONTEXT] to eval_input.
Every eval_input was working before because we forgot to restore MAIN_CONTEXT, so MAIN_CONTEXT was always present.

Delete conf parameter from def run which is always identical to IRB.conf, so we don't have to pass that param to eval_input.

[tompng]

I added the second commit just in case to reduce unexpected MAIN_CONTEXT restored to nil in multithread which will cause IRB crash. When binding.irb is executed in job-controlled multithread, non-nested binding.irb may start and end in random order.
Even with the second commit, there is a chance MAIN_CONTEXT being nil or wrong because we don't use mutex or thread local variable yet. (Example: uncontrolled multithread binding.irb)

Example of job-controlled multiple binding.irb.

def current
  [self, IRB.CurrentContext.main]
end
def pass(thread_id = nil)
  thread_id ||= next_thread_id
  raise "No such thread #{thread_id}" unless $queues[thread_id]
  $queues[thread_id].enq :pass
  $queues[$threads.index(Thread.current)].deq
  nil
end
def next_thread_id
  thread_id = $threads.index(Thread.current)
  (1..4).map { (thread_id + _1) % $queues.size }.find { $queues[_1] }
end
$queues = 4.times.map { Queue.new }
$threads = $queues.map.with_index do |queue, thread_id|
  "Thread #{thread_id}".instance_eval do
    Thread.new do
      queue.deq
      binding.irb
      $queues[thread_id] = nil
      $queues[next_thread_id].enq :pass if next_thread_id
    end
  end
end
$queues[0].enq :pass
$threads.map(&:join)

$ ruby file.rb
irb(Thread 0):001> pass 1
irb(Thread 1):001> pass 2
irb(Thread 2):001> pass 3
irb(Thread 3):001> pass 0
irb(Thread 0):002> exit # This will restore MAIN_CONTEXT to nil
irb(Thread 1):002> pass 3
irb(Thread 3):002> exit
irb(Thread 1):003> pass 2
irb(Thread 2):002> exit
irb(Thread 1):004> exit
# Order: start 0, start 1, start 2, start 3, exit 0, exit 3, exit 2, exit 1

[tompng]

Restoring to nil may crash IRB.
I'll create more simple pull request first, and re-think later for fundamental solution.