refactor: Use NotNull for CreateNewBlock and pimpls #36451

Current code uses nullable smart pointers for some stuff that is never null. This is perfectly fine, but a bit confusing:

• When reading the code, it is not clear which pointer is nullable or never null.
• Pointers that are never null still have confusing null checks (with possible error handling return branches), which are always dead in coverage reports or when applying mutation testing.
• While some places have the redundant null checks on pointers that are never null, other places do not, which adds to the confusion.

Clarify this at compile-time by using NotNull (or a plain reference)

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36451
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36451.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36451#author-ralph

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline and AI policy for information on the review process.

If your review is incorrectly listed, please copy-paste <code>&lt;!--meta-tag:bot-skip--&gt;</code> into the comment that the bot should ignore.

<!--174a7506f384e20aa4161008e828411d-->

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #36357 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36357.svg">} (mining: move BlockAssembler statics into BlockTemplateManager by w0xlt)
• #36356 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36356.svg">} (mining: move getblocktemplate cache into BlockTemplateManager by w0xlt)
• #35671 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/35671.svg">} (mining: add TxCollection to bandwidth-efficiently validate external block templates by Sjors)
• #33922 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/33922.svg">} (mining: add getMemoryLoad() and track template non-mempool memory footprint by Sjors)
• #33117 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/33117.svg">} (Interfaces: Expose UTXO Snapshot Loading and Add Progress Notifications by D33r-Gee)
• #32468 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/32468.svg">} (rpc: generateblock to allow multiple outputs by polespinasa)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

<!--5faf32d7da4f0f540f40219e4f7537a3-->

Concept ACK

The Windows MSVC bugs are a bit tiring. Can someone remove it? Ref: https://github.com/bitcoin/bitcoin/issues/36153

Concept ACK

The Windows MSVC bugs are a bit tiring.

Here is an MRE: https://godbolt.org/z/Kadaq5h46.

However, https://en.cppreference.com/cpp/compiler_support/20 lists P1825R0 as implemented in MSVC since v19.24 VS 2019 16.4.