Support Fiber#transfer-based fiber schedulers #1604

Job	Run time
test (Gemfile, 3.2)	54s
test (gemfiles/rails_4.2.gemfile, 2.4, 1)	59s
test (gemfiles/rails_5.2.gemfile, 2.7)	1m 15s
test (gemfiles/rails_7.0.gemfile, 3.1)	51s
test (gemfiles/rails_master.gemfile, 3.1)	55s
test (gemfiles/rails_7.0.gemfile, 3.2)	1m 4s
test (gemfiles/rails_master.gemfile, 3.2)	1m 43s
javascript_test	1m 32s
lint	44s
mongodb_test (gemfiles/mongoid_6.gemfile)	1m 26s
mongodb_test (gemfiles/mongoid_7.gemfile)	1m 8s
postgres_test	1m 43s
system_tests	1m 2s
	15m 16s

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