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

I wrote:

> Seems fine at a glance, thanks. I will build on this to implement variable-length values. I have already finished one prerequisite which is: public APIs passing pointers to values.

Since my publishing schedule has not kept up, I'm just going to share
something similar to what I mentioned earlier, just to get things
moving again.

0001-0009 are from earlier versions, except for 0007 which makes a
bunch of superficial naming updates, similar to those done in a recent
other version. Somewhere along the way I fixed long-standing git
whitespace warnings, but I don't remember if that's new here. In any
case, let's try to preserve that.

0010 is some minor refactoring to reduce duplication

0011-0014 add public functions that give the caller more control over
the input and responsibility for locking. They are not named well, but
I plan these to be temporary: They are currently used for the tidstore
only, since that has much simpler tests than the standard radix tree
tests. One thing to note: since the tidstore has always done it's own
locking within a larger structure, these patches don't bother to do
locking at the radix tree level. Locking twice seems...not great.
These patches are the main prerequisite for variable-length values.
Once that is working well, we can switch the standard tests to the new
APIs.

Next steps include (some of these were briefly discussed off-list with
Sawada-san):

- template parameter for varlen values
- some callers to pass length in bytes
- block entries to have num_elems for # of bitmap words
- a way for updates to re-alloc values when needed
- aset allocation for values when appropriate