Re: [PoC] Improve dead tuple storage for lazy vacuum

I wrote:

> Secondly, I thought about my recent work to skip checking if we first
> need to create a root node, and that has a harmless (for vacuum at
> least) but slightly untidy behavior: When RT_SET is first called, and
> the key is bigger than 255, new nodes will go on top of the root node.
> These have chunk '0'. If all subsequent keys are big enough, the
> orginal root node will stay empty. If all keys are deleted, there will
> be a chain of empty nodes remaining. Again, I believe this is
> harmless, but to make tidy, it should easy to teach RT_EXTEND_UP to
> call out to RT_EXTEND_DOWN if it finds the tree is empty. I can work
> on this, but likely not today.

This turns out to be a lot trickier than it looked, so it seems best
to allow a trivial amount of waste, as long as it's documented
somewhere. It also wouldn't be terrible to re-add those branches,
since they're highly predictable.

I just noticed there are a lot of unused function parameters
(referring to parent slots) leftover from a few weeks ago. Those are
removed in v64-0009. 0010 makes the obvious name change in those
remaining to "parent_slot". 0011 is a simplification in two places
regarding reserving slots. This should be a bit easier to read and
possibly makes it easier on the compiler.