refactor directory structure #292

Job	Run time
test_matrix (ubuntu, 2.5)	27s
test_matrix (ubuntu, 2.6)	26s
test_matrix (ubuntu, 2.7)	28s
test_matrix (ubuntu, 3.0)	19s
test_matrix (ubuntu, 3.1)	17s
test_matrix (ubuntu, 3.2)	14s
test_matrix (ubuntu, head)	14s
test_matrix (ubuntu, truffleruby)	38s
test_matrix (ubuntu, truffleruby-head)	50s
test_matrix (ubuntu, jruby)	2m 22s
test_matrix (ubuntu, jruby-head)	1m 9s
test_matrix (macos, 2.5)	51s
test_matrix (macos, 2.6)	55s
test_matrix (macos, 2.7)	56s
test_matrix (macos, 3.0)	21s
test_matrix (macos, 3.1)	19s
test_matrix (macos, 3.2)	21s
test_matrix (macos, head)	23s
test_matrix (macos, truffleruby)	1m 2s
test_matrix (macos, truffleruby-head)	1m 3s
test_matrix (macos, jruby)	1m 9s
test_matrix (macos, jruby-head)	1m 6s
finish	0s
	15m 50s

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