Use prototype declarations #10

Job	Run time
ruby-versions / Generate Ruby versions	1s
test (2.5, ubuntu-latest)	35s
test (2.5, macos-latest)	1m 8s
test (2.5, windows-latest)	2m 26s
test (2.6, ubuntu-latest)	36s
test (2.6, macos-latest)	1m 18s
test (2.6, windows-latest)	5m 21s
test (2.7, ubuntu-latest)	46s
test (2.7, macos-latest)	1m 19s
test (2.7, windows-latest)	2m 49s
test (3.0, ubuntu-latest)	25s
test (3.0, macos-latest)	44s
test (3.0, windows-latest)	1m 39s
test (3.1, ubuntu-latest)	27s
test (3.1, macos-latest)	42s
test (3.1, windows-latest)	2m 56s
test (3.2, ubuntu-latest)	26s
test (3.2, macos-latest)	41s
test (3.2, windows-latest)	4m 2s
test (head, ubuntu-latest)	28s
test (head, macos-latest)	50s
test (head, windows-latest)	2m 29s
test (jruby, ubuntu-latest)	5m 52s
test (jruby-head, ubuntu-latest)	7m 24s
test (truffleruby, ubuntu-latest)	2m 23s
test (truffleruby-head, ubuntu-latest)	4m 22s
	52m 9s

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