gh-131238: Move pycore_obmalloc.h include to pycore_runtime_structs.h (#131482)

Move pycore_obmalloc.h include from pycore_interp_structs.h to
pycore_runtime_structs.h.

Add also comment explaining the purpose of each include in
pycore_interp_structs.h, pycore_runtime_structs.h and
pycore_structs.h.

Remove <stdbool.h> and <stddef.h> from pycore_structs.h.