util/*FifoBuffer: migrate from WritableBuffer to std::span

src/client/Response.cxx

@@ -53,13 +53,13 @@ Response::VFmt(fmt::string_view format_str, fmt::format_args args) noexcept
 }
 
 bool
-Response::WriteBinary(ConstBuffer<void> payload) noexcept
+Response::WriteBinary(std::span<const std::byte> payload) noexcept
 {
-	assert(payload.size <= client.binary_limit);
+	assert(payload.size() <= client.binary_limit);
 
 	return
-		Fmt("binary: {}\n", payload.size) &&
-		Write(payload.data, payload.size) &&
+		Fmt("binary: {}\n", payload.size()) &&
+		Write(payload.data(), payload.size()) &&
 		Write("\n");
 }
 

src/client/Response.hxx

@@ -28,8 +28,8 @@
 #endif
 
 #include <cstddef>
+#include <span>
 
-template<typename T> struct ConstBuffer;
 class Client;
 class TagMask;
 
@@ -99,7 +99,7 @@ public:
 	 *
 	 * @return true on success
 	 */
-	bool WriteBinary(ConstBuffer<void> payload) noexcept;
+	bool WriteBinary(std::span<const std::byte> payload) noexcept;
 
 	void Error(enum ack code, const char *msg) noexcept;