This decreases the surface area of the block implementation in
preparation for deeper changes to its implementation.
See #98096
- Remove dead member functions.
- Remove last() check from next(), as described in its comment.
- Rework object lifetimes such that only block headers are actually
considered live. This simplifies their implementation.
- The allocated storage becomes live at the outer call to malloc-family
functions via a special case in the C++ standard.
- Add asserts for flag properties required by the implementation.
- Remove static from member functions that don't invalidate the block.
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