wallet: add wallet2_basic library types

The `wallet2_basic` library is a minimalistic library for loading / storing historic `wallet2` files. It has several design goals:
* Enable future migration away from monolithic `wallet2` codebase without losing funds
* Simplify FCMP++/Carrot integration
* Speed-up compile-time for current codebase, but especially the FCMP++/Carrot integration, which is split over several translation units

Author: jeffro256

src/wallet/CMakeLists.txt

@@ -104,3 +104,4 @@ if(NOT IOS)
 endif()
 
 add_subdirectory(api)
+add_subdirectory(wallet2_basic)

src/wallet/wallet2.cpp

@@ -93,6 +93,8 @@ using namespace epee;
 #include "device/device_cold.hpp"
 #include "device_trezor/device_trezor.hpp"
 #include "net/socks_connect.h"
+#include "wallet2_basic/wallet2_boost_serialization.h"
+#include "wallet2_basic/wallet2_serialization.h"
 
 extern "C"
 {