We are going to include generated small_config.h into small allocator
headers (currently it is only included in small source files).
core/memory.h depends on small headers and salad/heap.h depends on
core/memory.h. As a result we need to provide a way for salad/heap.h
users to find small_config.h header.

Instead let's drop dependency from core/memory.h as we only use it for
typeof definition.

Part of #7327

NO_CHANGELOG=code cleanup
NO_DOC=code cleanup

(cherry picked from commit d01609a4)