test: Roundtrip z_from_p x p_from_z #603

Job	Run time
test_c_api (macos-latest)	1m 36s
test (beta)	32s
test_c_api (windows-latest)	2m 8s
test (stable)	26s
test_c_api (ubuntu-latest)	23s
test (windows)	2m 14s
Check	8s
test (macos)	27s
Check if cbindgen runs cleanly for generating the C headers	23s
coverage	54s
cross_testing (aarch64-unknown-linux-gnu)	1m 37s
Lints	35s
cross_testing (powerpc64le-unknown-linux-gnu)	1m 27s
cross_testing (s390x-unknown-linux-gnu)	1m 35s
minimum_rust_version	1m 52s
Publish (dry-run)	6s
	16m 23s

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