Allow racc cmdline to read from stdin if no path specified. #58

Job	Run time
ruby-versions / Generate Ruby versions	3s
test (2.5, ubuntu-latest)	26s
test (2.5, macos-latest)	1m 29s
test (2.5, windows-latest)	2m 36s
test (2.6, ubuntu-latest)	28s
test (2.6, macos-latest)	1m 26s
test (2.6, windows-latest)	2m 59s
test (2.7, ubuntu-latest)	31s
test (2.7, macos-latest)	1m 9s
test (2.7, windows-latest)	1m 57s
test (3.0, ubuntu-latest)	20s
test (3.0, macos-latest)	30s
test (3.0, windows-latest)	1m 50s
test (3.1, ubuntu-latest)	20s
test (3.1, macos-latest)	29s
test (3.1, windows-latest)	1m 48s
test (3.2, ubuntu-latest)	21s
test (3.2, macos-latest)	37s
test (3.2, windows-latest)	2m 3s
test (3.3, ubuntu-latest)	25s
test (3.3, macos-latest)	39s
test (3.3, windows-latest)	3m 37s
test (head, ubuntu-latest)	28s
test (head, macos-latest)	40s
test (head, windows-latest)	2m 10s
test (jruby, ubuntu-latest)	4m 27s
test (jruby-head, ubuntu-latest)	4m 32s
test (truffleruby, ubuntu-latest)	2m 8s
test (truffleruby-head, ubuntu-latest)	2m 48s
	43m 16s

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