Avoid double allocating while hashing&serializing

[dvdplm]

Avoid the serialize-then-hash pattern by adding a HashingWriter that impls std::io::Writer. Now we can avoid passing a Vec buffer to safe_serialize and then hash that Vec; instead the HashingWriter hashes as it goes and finalizes in the end.

The performance impact is very minor, sadly, most likely because serialization still allocates a bunch. On my system I see memory consumption for one big test go down from ~9Gb to ~8Gb, so not useless I guess.

WIP because it's worth checking other call sites for the "serialize-then-hash" pattern.

[github-actions]

Consolidated Tests Results 2026-03-31 - 17:13:27

Test Results

21 passed

Details

21 tests
not captured
junit-to-ctrf
build-and-test test-reporter #1110
Avoid double allocating while hashing&serializing #496

test-reporter: Run #1110

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Pending ⏳	Other ❓	Flaky 🍂	Duration ⏱️
21	21	0	0	0	0	0	not captured

🎉 All tests passed!

Tests

View All Tests

Test Name	Status	Flaky	Duration
nightly_full_gen_tests_k8s_default_threshld_sequential_crs	✅		33.2s
test_k8s_threshld_insecure	✅		3m 19s
k8s_test_crs_uniqueness	✅		33.1s
k8s_test_insecure_keygen_encrypt_and_public_decrypt	✅		3m 23s
k8s_test_insecure_keygen_encrypt_multiple_types	✅		3m 44s
k8s_test_keygen_and_crs	✅		3m 19s
k8s_test_keygen_uniqueness	✅		9m 4s
nightly_full_gen_tests_k8s_default_threshld_sequential_crs	✅		33.2s
test_k8s_threshld_insecure	✅		3m 19s
k8s_test_crs_uniqueness	✅		33.1s
k8s_test_insecure_keygen_encrypt_and_public_decrypt	✅		3m 23s
k8s_test_insecure_keygen_encrypt_multiple_types	✅		3m 44s
k8s_test_keygen_and_crs	✅		3m 19s
k8s_test_keygen_uniqueness	✅		9m 4s
nightly_full_gen_tests_k8s_default_threshld_sequential_crs	✅		33.2s
test_k8s_threshld_insecure	✅		3m 19s
k8s_test_crs_uniqueness	✅		33.1s
k8s_test_insecure_keygen_encrypt_and_public_decrypt	✅		3m 23s
k8s_test_insecure_keygen_encrypt_multiple_types	✅		3m 44s
k8s_test_keygen_and_crs	✅		3m 19s
k8s_test_keygen_uniqueness	✅		9m 4s

_{🍂 No flaky tests in this run.}

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