[tiptenbrink]

Stacked on #53. Every function that writes a request now returns a WireRequest that borrows nothing. It holds the method, and where the URL, head, field lines and body are in the buffer. Each part is read by passing that buffer. A host that keeps its buffer can grow it and write a request again without converting anything:

let request = crate::grown_to_fit(&mut self.buf, |buf| {
    azure::encode_get(template, bytes, &item, &now, buf)
})?;
let (parts, body) = crate::send(&self.agent, &request, &self.buf, None)?;

Before, an encoder returned a WireRequest<'r> that borrowed the buffer, and the caller's content. A host that grew its buffer had to add .map(|request| request.placement()) to each call, and turn the Placement back into a request with placement.request(&buf).

What changes

• WireRequest has no lifetime and is Copy. url(&buf), headers(&buf), head(&buf) and payload(&buf) read its parts from the buffer it was written into. The span accessors (url_span, head_span, fields_span, body_span) stay, with a new content_len.
• Placement and WireRequest::placement are gone. The C ABI already had only the span struct, borink_wire_request, so the Rust and C types now match.
• Every azure::encode_*, s3::set_signature and sigv4::sign returns a WireRequest. An S3 encoder still returns an s3::Unsigned, which no longer has a lifetime. Its canonical(&buf) and body(&buf) already read the buffer the same way.
• A request no longer lends back content passed as Payload::Slice. It states the content's length as Payload::Streamed, and the host sends the bytes it holds. A body that the crate wrote, such as a block list or a list of parts, is still lent from the buffer. A request without content still has an empty Payload::Slice.
• Passing the wrong buffer gives a wrong request, but never a panic, as before.

Callers

• The ureq host's send takes the request and its buffer.
• The object-tests host sends a (WireRequest, &[u8]) pair. It also names each Azure encoder once: one closure sizes the buffer and writes the request.
• The C ABI converts a WireRequest to its request struct. The C types and the header are unchanged.
• azure_put's content test now expects held and streamed content to give the same request.

Not in this PR: object-storage.dev's replay and benchmarks, which read WireRequest through the borrowing accessors. They move to the new readers when they next take this crate.

Checked locally: cargo fmt --check, clippy on all targets and features, every workspace test, the docs with -D warnings, and the link check without an allocator.