Design Task: T76372
This part of a larger refactoring towards a more extensible architecture
in Blender: T75724
The API is defined in `BLO_read_write.h`. It adds the small data structures
`BlendWriter`, `BlendDataReader`, `BlendLibReader` and `BlendExpander`.
Those contain context about the current read/write operation. Furthermore,
it adds many functions with the prefixes `BLO_write_*`, `BLO_read_*` and
`BLO_expand_*`.
Lib linking and expanding will probably be handled by the more generic libquery
system eventually. The corresponding parts of the API can be removed then.
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