[ruby/irb] Fix nested IRB sessions' history saving

(https://github.com/ruby/irb/pull/652)

1. Dynamically including `HistorySavingAbility` makes things unnecessarily
   complicated and should be avoided.
2. Because both `Reline` and `Readline` use a single `HISTORY` constant
   to store history data. When nesting IRB sessions, only the first IRB
   session should handle history loading and saving so we can avoid
   duplicating history.
3. History saving callback should NOT be stored in `IRB.conf` as it's
   recreated every time `IRB.setup` is called, which would happen when
   nesting IRB sessions.

https://github.com/ruby/irb/commit/0fef0ae160

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