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/SField.h

@@ -120,8 +120,8 @@ field_code(int id, int index)
     SFields are created at compile time.
 
     Each SField, once constructed, lives until program termination, and there
-    is only one instance per fieldType/fieldValue pair which serves the entire
-    application.
+    is only one instance per fieldType/fieldValue pair which serves the
+    entire application.
 */
 class SField
 {