Borrow function arguments and use one byte type #1987

Function calls copy byte and string arguments more often than they need to.

Problem

• call and call_host take arguments by value. A caller that holds a &str or &[u8] must clone it first, even though the call only reads it.
• A function can receive bytes as Vec<u8> or Vec<Bytes>. In the guest, Vec<u8> joins the transport buffers into a new vector and Vec<Bytes> does not. The cost is hidden in the receiver's signature.

Proposal

Callers pass borrowed values. Functions take and return bytes only as Vec<Bytes>.

Arguments are what callers pass to call, call_host and #[host_function] stubs. Parameters and return types are what #[guest_function] and host functions declare.

Breaking changes

• Owned String, Vec<u8>, Bytes and Vec<Bytes> are no longer accepted as arguments. Pass s.as_str(), &v or &chunks.
• Vec<u8> is no longer a parameter or return type. Use Vec<Bytes>. concat() joins the chunks when a function needs one contiguous buffer.

These are hard breaks, but each fix is a one-line change at a call site or a function signature, and the compile error names it.

Effect

Copies per byte argument, host to guest:

After the change, every byte argument takes one copy: the host writes it into the guest's transport buffers, and the guest reads it from them in place. Guest-to-host calls save the caller's clone. The host still copies guest data once before reading it, for safety.