Add `{save,restore}_checkpoint` to `PartialPaths` to rewind the arena… #25

Job	Run time
test-cli (stable)	1m 54s
list-languages	4s
test-rust (stable)	5m 6s
test-languages (tree-sitter-stack-graphs-typescript, stable)	2m 31s
test-languages (tree-sitter-stack-graphs-java, stable)	1m 20s
test-languages (tree-sitter-stack-graphs-javascript, stable)	1m 27s
test-languages (tree-sitter-stack-graphs-python, stable)	1m 22s
test-init (stable)	35s
	14m 19s

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