Fuzz seeds

One directory per fuzz target, named as the target is (npro_fuzz::Target). Every file in it is one input libFuzzer starts its corpus from, and one the smoke tests in crates/npro-fuzz/tests/smoke.rs run on every cargo test. Files named *.md are left out of both.

Where a target splits its input to feed it in pieces, as utf8 and sha1 do, the first byte chooses the split and the bytes under test follow it. A seed for those targets starts with that byte; any value will do.

The transcript target also starts from every transcript in crates/npro-test/transcripts/.

Seeds are small and readable on purpose: each is a case worth starting from, named for what it is. What the fuzzer finds goes in its corpus, which is kept between runs, not here (docs/fuzzing.md). When the protocol crates arrive, their targets' seeds come from the C library's corpora (fuzz/fuzz-*/seeds in the C tree), copied with where they came from.