perf: Make STVar, STObject and STArray moves noexcept

std::vector moves elements on reallocation only if the move
constructor is noexcept; otherwise it copies them. These move
constructors were not noexcept, so every time STObject::v_ or
STArray::v_ grew, each existing element was deep-copied through the
virtual STBase::copy. Add static_asserts so this does not regress.

include/xrpl/protocol/STArray.h

@@ -42,9 +42,9 @@ public:
 
     STArray&
     operator=(STArray const&) = default;
-    STArray(STArray&&);
+    STArray(STArray&&) noexcept;
     STArray&
-    operator=(STArray&&);
+    operator=(STArray&&) noexcept;
 
     STArray(SField const& f, std::size_t n);
     STArray(SerialIter& sit, SField const& f, int depth = 0);

include/xrpl/protocol/STObject.h

@@ -85,9 +85,9 @@ public:
 
     STObject&
     operator=(STObject const&) = default;
-    STObject(STObject&&);
+    STObject(STObject&&) noexcept;
     STObject&
-    operator=(STObject&& other);
+    operator=(STObject&& other) noexcept;
 
     STObject(SOTemplate const& type, SField const& name);
     STObject(

include/xrpl/protocol/detail/STVar.h

@@ -45,11 +45,11 @@ private:
 public:
     ~STVar();
     STVar(STVar const& other);
-    STVar(STVar&& other);
+    STVar(STVar&& other) noexcept;
     STVar&
     operator=(STVar const& rhs);
     STVar&
-    operator=(STVar&& rhs);
+    operator=(STVar&& rhs) noexcept;
 
     // NOLINTNEXTLINE(cppcoreguidelines-rvalue-reference-param-not-moved)
     STVar(STBase&& t) : p_(t.move(kMaxSize, &d_))

src/libxrpl/protocol/STArray.cpp

@@ -11,16 +11,20 @@
 #include <cstddef>
 #include <stdexcept>
 #include <string>
+#include <type_traits>
 #include <utility>
 
 namespace xrpl {
 
-STArray::STArray(STArray&& other) : STBase(other.getFName()), v_(std::move(other.v_))
+// Lets std::vector move, not copy, elements when it grows.
+static_assert(std::is_nothrow_move_constructible_v<STArray>);
+
+STArray::STArray(STArray&& other) noexcept : STBase(other.getFName()), v_(std::move(other.v_))
 {
 }
 
 STArray&
-STArray::operator=(STArray&& other)
+STArray::operator=(STArray&& other) noexcept
 {
     setFName(other.getFName());
     v_ = std::move(other.v_);

src/libxrpl/protocol/STObject.cpp

@@ -37,12 +37,16 @@
 #include <sstream>
 #include <stdexcept>
 #include <string>
+#include <type_traits>
 #include <utility>
 #include <vector>
 
 namespace xrpl {
 
-STObject::STObject(STObject&& other)
+// Lets std::vector move, not copy, elements when it grows.
+static_assert(std::is_nothrow_move_constructible_v<STObject>);
+
+STObject::STObject(STObject&& other) noexcept
     : STBase(other.getFName()), v_(std::move(other.v_)), type_(other.type_)
 {
 }
@@ -126,7 +130,7 @@ STObject::add(Serializer& s) const
 }
 
 STObject&
-STObject::operator=(STObject&& other)
+STObject::operator=(STObject&& other) noexcept
 {
     setFName(other.getFName());
     type_ = other.type_;

src/libxrpl/protocol/STVar.cpp

@@ -25,6 +25,9 @@
 
 namespace xrpl::detail {
 
+// Lets std::vector move, not copy, elements when it grows.
+static_assert(std::is_nothrow_move_constructible_v<STVar>);
+
 DefaultObjectT gDefaultObject;
 NonPresentObjectT gNonPresentObject;
 
@@ -41,7 +44,7 @@ STVar::STVar(STVar const& other)
         p_ = other.p_->copy(kMaxSize, &d_);
 }
 
-STVar::STVar(STVar&& other)
+STVar::STVar(STVar&& other) noexcept
 {
     if (other.onHeap())
     {
@@ -74,7 +77,7 @@ STVar::operator=(STVar const& rhs)
 }
 
 STVar&
-STVar::operator=(STVar&& rhs)
+STVar::operator=(STVar&& rhs) noexcept
 {
     if (&rhs != this)
     {