Clean up dependencies in memoryview utility code (GH-7210)

• Move __Pyx_init_memviewslice() out of the general declarations to make it a dependency.
• Move refcounting related declarations into a separate section.
• Avoid a circular dependency for the MemviewSliceCopyTemplate utility code.
• Use Naming substitutions throughout to reduce the Tempita runs.
• Remove the unused __Pyx_BUF_MAX_NDIMS macro.
• Fix “_Py_NO_RETURN” macro usage (only used as a hint).