This does introduce some not-so-nice things, like having to annotate each
`__init__` function with `-> None`. However, the benefits of having static
type checking in a complex bit of software like BAT outweigh the downsides.
Due to the huge number of BlendFileBlock objects created for packing a
production-size blend file, using slots here actually makes the
dependency tracer significantly (p<0.001) faster. In my test case the
speed improvement was 16% for a 'bam list' command.
This simplifies blend file handling, ensuring that blend files are only
opened once. Otherwise it would be harder to handle things like dependency
diamonds (libraries that are referenced via multiple paths through multiple
other libraries).
The expansion process follows pointers and library links to construct
the full set of actually-used data blocks. This set consists of all data
blocks in the initial blend file, and all *actually linked-to* data
blocks in linked blend files.
I've also removed non-recursive dependency listing.
Evaluated only once, so safe to call multiple times without producing
excessive disk I/O. Returns None instead of raising KeyError when there is
no (b'id', b'name') field.
Type refinement is now only done with BlendFileBlock.refine_type(), and
no longer with sdna_index_refine parameters to various functions. This
simplifies the API at the expense of having to call two simple functions
instead of one more complex one.
