Blender Git Loki

Cleanup: Assets, move AssetLibrary struct to global namespace

Move `blender::bke::AssetLibrary` to the global namespace, causing the
C `struct AssetLibrary` to refer to the same object as the corresponding C++
struct.

Also move `intern/asset_library.hh` to `BKE_asset_library.hh`,
mimicking `BKE_geometry_set.hh`.

This makes the C and C++ pointers compatible, cleans up the code a bit,
and aids in debugging.

No functional changes.