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/libxrpl/protocol/STNumber.cpp

@@ -0,0 +1,105 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2023 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/protocol/STNumber.h>
+
+#include <xrpl/protocol/SField.h>
+
+namespace ripple {
+
+STNumber::STNumber(SField const& field, Number const& value)
+    : STBase(field), value_(value)
+{
+}
+
+STNumber::STNumber(SerialIter& sit, SField const& field) : STBase(field)
+{
+    // We must call these methods in separate statements
+    // to guarantee their order of execution.
+    auto mantissa = sit.geti64();
+    auto exponent = sit.geti32();
+    value_ = Number{mantissa, exponent};
+}
+
+SerializedTypeID
+STNumber::getSType() const
+{
+    return STI_NUMBER;
+}
+
+std::string
+STNumber::getText() const
+{
+    return to_string(value_);
+}
+
+void
+STNumber::add(Serializer& s) const
+{
+    assert(getFName().isBinary());
+    assert(getFName().fieldType == getSType());
+    s.add64(value_.mantissa());
+    s.add32(value_.exponent());
+}
+
+Number const&
+STNumber::value() const
+{
+    return value_;
+}
+
+void
+STNumber::setValue(Number const& v)
+{
+    value_ = v;
+}
+
+STBase*
+STNumber::copy(std::size_t n, void* buf) const
+{
+    return emplace(n, buf, *this);
+}
+
+STBase*
+STNumber::move(std::size_t n, void* buf)
+{
+    return emplace(n, buf, std::move(*this));
+}
+
+bool
+STNumber::isEquivalent(STBase const& t) const
+{
+    assert(t.getSType() == this->getSType());
+    STNumber const& v = dynamic_cast<STNumber const&>(t);
+    return value_ == v;
+}
+
+bool
+STNumber::isDefault() const
+{
+    return value_ == Number();
+}
+
+std::ostream&
+operator<<(std::ostream& out, STNumber const& rhs)
+{
+    return out << rhs.getText();
+}
+
+}  // namespace ripple