[DGonzalezVillal]

Adds the sev-core crate from the workspace refactor proposal (docs/design/workspace-crates.md). Crate version is 0.1.0.

What's in it

• parser: Cursor over &[u8], Decoder<P> and Encoder<P>, ByteParser (from_bytes/to_bytes), ByteWrite for Vec<u8>, and ParseError. Skips are zero-checked (skip_bytes) or return the original region (skip_bytes_lax).
• Authenticate: a key or certificate that checks whether a signature belongs to it.
• Verifiable<Root>: an artifact that verifys against a root. verify calls root.authenticate and returns an associated Output.
• Attestable: produces an attestation token from a verified artifact.
  No crypto, no ABI types, and no role-crate impls. Those belong in sev-types, sev-endorser, and sev-verifier.

Testing

• cargo test -p sev-core: 25 unit tests, 5 doc tests
• cargo clippy -p sev-core --all-targets -- -D clippy::all -D unused_imports -D warnings -D clippy::style
• cargo fmt --all -- --check

Open questions

• Authenticate::authenticate takes only the signature. A signature check on a report also needs the signed body, so the report impl of Verifiable is not included here.