refactor directory structure #293

Job	Run time
test_matrix (ubuntu, 2.5)	32s
test_matrix (ubuntu, 2.6)	28s
test_matrix (ubuntu, 2.7)	30s
test_matrix (ubuntu, 3.0)	13s
test_matrix (ubuntu, 3.1)	38s
test_matrix (ubuntu, 3.2)	16s
test_matrix (ubuntu, head)	10s
test_matrix (ubuntu, truffleruby)	40s
test_matrix (ubuntu, truffleruby-head)	1m 3s
test_matrix (ubuntu, jruby)	50s
test_matrix (ubuntu, jruby-head)	46s
test_matrix (macos, 2.5)	1m 8s
test_matrix (macos, 2.6)	53s
test_matrix (macos, 2.7)	50s
test_matrix (macos, 3.0)	17s
test_matrix (macos, 3.1)	29s
test_matrix (macos, 3.2)	25s
test_matrix (macos, head)	23s
test_matrix (macos, truffleruby)	1m 4s
test_matrix (macos, truffleruby-head)	1m 11s
test_matrix (macos, jruby)	57s
test_matrix (macos, jruby-head)	55s
finish	0s
	14m 38s

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