fixup! b_tree: Add some method-level documentation.
ut-blocks: Amend erase test.
b_tree: Fix block fuse code.
b_tree: Support changing the fanout.
ut-blocks erase: Add --fanout switch.
Add free_space_bitmap speed test.
block_collection: Pack more bits into the free space bitmap.

Introduce a `free_space_block` class that manages the contents of a
`block_buffer` which contains the free space bitmap.