[EXPERIMENT]: Subscriptions: Fuse BSATN for all table row ops in one allocation

[Centril]

Description of Changes

Making a combined combined "ticket plus code experiment" that demonstrates the issue.
Here in this PR, instead of creating one Vec<u8> of BSATN per row, we put all the BSATN of all rows in a single Vec<u8>.
(In a real PR we'd need to add the number of elements as well and other changes, but this is just a perf demonstration.)

In a sample benchmark run on my i7-7700K, 64GB RAM, this result in the following improvements compared to HEAD~1:

Benchmarking full-scan: Collecting 100 samples in estimated 5.5034 s (200 iteration
full-scan               time:   [27.010 ms 27.122 ms 27.300 ms]
                        change: [-71.553% -71.319% -71.037%] (p = 0.00 < 0.05)
                        Performance has improved.

Benchmarking full-join: Collecting 100 samples in estimated 5.2570 s (25k iteration
full-join               time:   [216.69 µs 218.12 µs 220.19 µs]
                        change: [-25.202% -24.271% -23.234%] (p = 0.00 < 0.05)
                        Performance has improved.

For full-scan, this is about a 3.5x improvement to runtime (was about 96ms before).
For full-join, this is about a 1.3x improvement (full-join does more additional interesting work, explaining the difference).

Here is a picture of the flamegraph before:

And here is one after:

The big malloc bar is gone and instead, TableCursor + and to_bsatn_extend are taking up the remaining time.

API and ABI breaking changes

If this is an API or ABI breaking change, please apply the
corresponding GitHub label.

Expected complexity level and risk

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where 1 is a trivial change, and 5 is a deep-reaching and complex change.

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but also to its interactions with existing and future code.

If you answered more than a 2, explain what is complex about the PR,
and what other components it interacts with in potentially concerning ways.

Testing

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so that you're confident that all the changes work as expected!

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[Centril]

Blocked by #1077.