[Eva] Moves files from value_types into src/kernel_services/abstract_interp.

These files were previously in src/plugins/value_types, and very briefly
in src/kernel_externals/value_types.

src/kernel_externals/value_types/README.md

-This directory contains lattices representing the C memory model of Value,
-and some of its derived plugins (From, Inout).

src/kernel_services/abstract_interp/README.md

-This directory contains generic lattices for abstract interpretation.
+This directory contains generic lattices for abstract interpretation
+and the C memory model of Eva.
 
-Nowadays it is mainly used by the Value plug-in and its derived plug-ins.
+It is mainly used by Eva and its derived plug-ins (From, Inout).