Implement Lending Protocol (unsupported) (#5270)

- Spec: XLS-66
- Introduces amendment "LendingProtocol", but leaves it UNSUPPORTED to
  allow for standalone testing, future development work, and potential
  bug fixes.
- AccountInfo RPC will indicate the type of pseudo-account when
  appropriate.
- Refactors and improves several existing classes and functional areas,
  including Number, STAmount, STObject, json_value, Asset, directory
  handling, View helper functions, and unit test helpers.

include/xrpl/protocol/STObject.h

@@ -482,6 +482,8 @@ public:
     value_type
     operator*() const;
 
+    /// Do not use operator->() unless the field is required, or you've checked
+    /// that it's set.
     T const*
     operator->() const;
 
@@ -505,7 +507,26 @@ protected:
 // Constraint += and -= ValueProxy operators
 // to value types that support arithmetic operations
 template <typename U>
-concept IsArithmetic = std::is_arithmetic_v<U> || std::is_same_v<U, STAmount>;
+concept IsArithmeticNumber = std::is_arithmetic_v<U> ||
+    std::is_same_v<U, Number> || std::is_same_v<U, STAmount>;
+template <
+    typename U,
+    typename Value = typename U::value_type,
+    typename Unit = typename U::unit_type>
+concept IsArithmeticValueUnit =
+    std::is_same_v<U, unit::ValueUnit<Unit, Value>> &&
+    IsArithmeticNumber<Value> && std::is_class_v<Unit>;
+template <typename U, typename Value = typename U::value_type>
+concept IsArithmeticST = !IsArithmeticValueUnit<U> && IsArithmeticNumber<Value>;
+template <typename U>
+concept IsArithmetic =
+    IsArithmeticNumber<U> || IsArithmeticST<U> || IsArithmeticValueUnit<U>;
+
+template <class T, class U>
+concept Addable = requires(T t, U u) { t = t + u; };
+template <typename T, typename U>
+concept IsArithmeticCompatible =
+    IsArithmetic<typename T::value_type> && Addable<typename T::value_type, U>;
 
 template <class T>
 class STObject::ValueProxy : public Proxy<T>
@@ -525,10 +546,12 @@ public:
     // Convenience operators for value types supporting
     // arithmetic operations
     template <IsArithmetic U>
+        requires IsArithmeticCompatible<T, U>
     ValueProxy&
     operator+=(U const& u);
 
     template <IsArithmetic U>
+        requires IsArithmeticCompatible<T, U>
     ValueProxy&
     operator-=(U const& u);
 
@@ -718,6 +741,8 @@ STObject::Proxy<T>::operator*() const -> value_type
     return this->value();
 }
 
+/// Do not use operator->() unless the field is required, or you've checked that
+/// it's set.
 template <class T>
 T const*
 STObject::Proxy<T>::operator->() const
@@ -764,6 +789,7 @@ STObject::ValueProxy<T>::operator=(U&& u)
 
 template <typename T>
 template <IsArithmetic U>
+    requires IsArithmeticCompatible<T, U>
 STObject::ValueProxy<T>&
 STObject::ValueProxy<T>::operator+=(U const& u)
 {
@@ -773,6 +799,7 @@ STObject::ValueProxy<T>::operator+=(U const& u)
 
 template <class T>
 template <IsArithmetic U>
+    requires IsArithmeticCompatible<T, U>
 STObject::ValueProxy<T>&
 STObject::ValueProxy<T>::operator-=(U const& u)
 {