Move hashtable to ds/ folder

Notably, remove the include_directories(), which is not necessary, as
most files do not actually use this.

common/utils/ds/CMakeLists.txt

@@ -2,6 +2,8 @@ add_library(ds OBJECT
             byte_array.c
             byte_array_producer.c
             seq_arr.c
+            hashtable.c
+            obj_hashtable.c
 )
 
 target_include_directories(ds PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})