[tiny-ac]

Reads from a REAPI server's content-addressable storage (CAS) and action cache. Nothing calls them yet; later PRs look compiles up in the action cache and download their outputs.

Cas downloads blobs over ByteStream, zstd-compressed when the server supports it, into memory or a temporary file. A blocking thread decompresses, hashes and writes each blob as it arrives, and the download fails if the blob's size or hash doesn't match its digest, or if the server sends more bytes than the digest allows. A blob the server doesn't have fails with MissingBlob, so callers can treat it as a cache miss.

ActionCache looks an action's result up, and returns None on a miss.

Changes

• src/reapi/cas.rs: Cas and MissingBlob.
• src/reapi/action_cache.rs: ActionCache.
• src/reapi/fake.rs: an in-memory REAPI server for tests, with Capabilities, ByteStream reads and the action cache.
• src/reapi/client.rs: RPC_TIMEOUT is visible to the other reapi modules.

Cost

Without the reapi feature, nothing new is compiled. With it, no new dependencies.

Validation

• fmt, and clippy with -D warnings, with and without reapi.
• cargo test --features reapi --lib reapi::: the new tests run against the fake server, including corrupt, oversized and invalid blobs, with and without zstd, and a missing blob reported when the read starts or in its stream.
• cargo check --no-default-features --features reapi.