Add a new serialized type: STNumber (#5121)

`STNumber` lets objects and transactions contain multiple fields for
quantities of XRP, IOU, or MPT without duplicating information about the
"issue" (represented by `STIssue`). It is a straightforward serialization of
the `Number` type that uniformly represents those quantities.

---------

Co-authored-by: John Freeman <jfreeman08@gmail.com>
Co-authored-by: Howard Hinnant <howard.hinnant@gmail.com>

src/test/protocol/Serializer_test.cpp

@@ -0,0 +1,69 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2024 Ripple Labs Inc.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include <xrpl/beast/unit_test.h>
+#include <xrpl/protocol/Serializer.h>
+
+#include <limits>
+
+namespace ripple {
+
+struct Serializer_test : public beast::unit_test::suite
+{
+    void
+    run() override
+    {
+        {
+            std::initializer_list<std::int32_t> const values = {
+                std::numeric_limits<std::int32_t>::min(),
+                -1,
+                0,
+                1,
+                std::numeric_limits<std::int32_t>::max()};
+            for (std::int32_t value : values)
+            {
+                Serializer s;
+                s.add32(value);
+                BEAST_EXPECT(s.size() == 4);
+                SerialIter sit(s.slice());
+                BEAST_EXPECT(sit.geti32() == value);
+            }
+        }
+        {
+            std::initializer_list<std::int64_t> const values = {
+                std::numeric_limits<std::int64_t>::min(),
+                -1,
+                0,
+                1,
+                std::numeric_limits<std::int64_t>::max()};
+            for (std::int64_t value : values)
+            {
+                Serializer s;
+                s.add64(value);
+                BEAST_EXPECT(s.size() == 8);
+                SerialIter sit(s.slice());
+                BEAST_EXPECT(sit.geti64() == value);
+            }
+        }
+    }
+};
+
+BEAST_DEFINE_TESTSUITE(Serializer, protocol, ripple);
+
+}  // namespace ripple