This changes the type of `new_id_to_item` from `GHash` to
`Map<ID *, BlendfileLinkAppendContextItem *>` which simplifies code. For this to work
nicely, `BlendfileLinkAppendContext` had to become a non-trivial type. Since it
was allocated with `calloc` before, I gave all members
zero-default-initializers.
This also changes `BKE_blendfile_link_append_context_new` so that it uses
`MEM_new` instead of allocating the `BlendfileLinkAppendContext` as part of the
memory arena, because that's the more common way to do it, and it does not seem
like saving this one allocation here makes a difference considering how this is
used.
Pull Request: https://projects.blender.org/blender/blender/pulls/124516
a473107489