MonitorMixin#new_cond can be called before mon_initialize, so we
need to initialize `@monitor` before it.

https://bugs.ruby-lang.org/issues/16255#note-4

Since https://github.com/ruby/ruby/pull/2576,
`new_cond` uses the Monitor object, not the receiver.

MonitorMixin::ConditionVariable#wait can be interrupted just after
Monitor#exit_for_cond. So implementation in C.

Delegate MonitorMixin#synchronize body to Monitor#synchronize.
It makes guarantee interrupt safe (because Monitor#synchronize is
written in C). I thought Ruby implementation is also safe, but I
got stuck failure <http://ci.rvm.jp/results/trunk_test@P895/2327639>
so that I introduce this fix to guarantee interrupt safe.

Recent monitor.rb has performance problem because of interrupt
handlers. 'Monitor#synchronize' is frequently used primitive
so the performance of this method is important.

This patch rewrite 'monitor.rb' with 'monitor.so' (C-extension)
and make it faster. See [Feature #16255] for details.

Monitor class objects are normal object which include MonitorMixin.
This patch introduce a Monitor class which is implemented on C
and MonitorMixin uses Monitor object as re-entrant (recursive)
Mutex. This technique improve performance because we don't need
to care atomicity and we don't need accesses to instance variables
any more on Monitor class.