Config: Flatten structures

Originally, Layer contained a std::map of Sections, which containted a std::map
containing the (key, value) pairs. Here we flattern this structure so that only
one std::map is required, reducing the number of indirections required and
vastly simplifying the code.

Source/Core/Common/CMakeLists.txt

@@ -6,7 +6,6 @@ set(SRCS
   Config/Config.cpp
   Config/ConfigInfo.cpp
   Config/Layer.cpp
-  Config/Section.cpp
   Crypto/AES.cpp
   Crypto/bn.cpp
   Crypto/ec.cpp