refactor directory structure #282

Job	Run time
test_matrix (ubuntu, 2.5)	28s
test_matrix (ubuntu, 2.6)	35s
test_matrix (ubuntu, 2.7)	28s
test_matrix (ubuntu, 3.0)	11s
test_matrix (ubuntu, 3.1)	14s
test_matrix (ubuntu, 3.2)	11s
test_matrix (ubuntu, head)	13s
test_matrix (ubuntu, truffleruby)	48s
test_matrix (ubuntu, truffleruby-head)	1m 2s
test_matrix (macos, 2.5)	1m 6s
test_matrix (macos, 2.6)	1m 15s
test_matrix (macos, 2.7)	1m 2s
test_matrix (macos, 3.0)	20s
test_matrix (macos, 3.1)	32s
test_matrix (macos, 3.2)	22s
test_matrix (macos, head)	19s
test_matrix (macos, truffleruby)	58s
test_matrix (macos, truffleruby-head)	1m 16s
finish	0s
	11m 20s

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