net/FormatAddress: new library to replace ToString.cxx

This library writes to a caller-specified buffer instead of
allocating a std::string which can be faster by avoiding heap
allocations.

src/net/FormatAddress.hxx

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// Copyright CM4all GmbH
+// author: Max Kellermann <mk@cm4all.com>
+
+#pragma once
+
+#include <cstddef>
+#include <span>
+
+class SocketAddress;
+
+/**
+ * Generates the string representation of a #SocketAddress into the
+ * specified buffer.
+ *
+ * @return true on success
+ */
+bool
+ToString(std::span<char> buffer, SocketAddress address) noexcept;
+
+/**
+ * Like ToString() above, but return the string pointer (or on error:
+ * return the given fallback pointer).
+ */
+const char *
+ToString(std::span<char> buffer, SocketAddress address,
+	 const char *fallback) noexcept;
+
+/**
+ * Generates the string representation of a #SocketAddress into the
+ * specified buffer, without the port number.
+ *
+ * @return true on success
+ */
+bool
+HostToString(std::span<char> buffer, SocketAddress address) noexcept;