Added block expansion

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.

blender_asset_tracer/blendfile/iterators.py

@@ -1,10 +1,39 @@
+import typing
+
+from blender_asset_tracer import cdefs
 from . import BlendFileBlock
 from .dna import FieldPath
 
 
-def listbase(block: BlendFileBlock, next_path: FieldPath=b'next') -> BlendFileBlock:
+def listbase(block: BlendFileBlock, next_path: FieldPath = b'next') \
+        -> typing.Iterator[BlendFileBlock]:
     """Generator, yields all blocks in the ListBase linked list."""
     while block:
         yield block
         next_ptr = block[next_path]
         block = block.bfile.find_block_from_address(next_ptr)
+
+
+def sequencer_strips(sequence_editor: BlendFileBlock) \
+        -> typing.Iterator[typing.Tuple[BlendFileBlock, int]]:
+    """Generator, yield all sequencer strip blocks with their type number.
+
+    Recurses into meta strips, yielding both the meta strip itself and the
+    strips contained within it.
+
+    See blender_asset_tracer.cdefs.SEQ_TYPE_xxx for the type numbers.
+    """
+
+    def iter_seqbase(seqbase) -> typing.Iterator[BlendFileBlock]:
+        for seq in listbase(seqbase):
+            seq.refine_type(b'Sequence')
+            seq_type = seq[b'type']
+            yield seq, seq_type
+
+            if seq_type == cdefs.SEQ_TYPE_META:
+                # Recurse into this meta-sequence.
+                subseq = seq.get_pointer((b'seqbase', b'first'))
+                yield from iter_seqbase(subseq)
+
+    sbase = sequence_editor.get_pointer((b'seqbase', b'first'))
+    yield from iter_seqbase(sbase)