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. 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.