refactor: remove unnecessary unsafe

[koivunej]

unsafe impls for Send and Sync should not be added by default. in the case of SlotGuard removing them does not cause any issues, as the compiler automatically derives those.

This PR adds requirement to document the unsafety (see clippy::undocumented_unsafe_blocks) and opportunistically adds #![deny(unsafe_code)] to most places where we don't have unsafe code right now.

TRPL on Send and Sync: https://doc.rust-lang.org/book/ch16-04-extensible-concurrency-sync-and-send.html

[jcsp]

Thanks for cleaning this up: that's my fault for adding some Send/Sync in some earlier variant of the code & not removing it later.

[koivunej]

(moving the comment for walredo pre_exec soon, checking out other unsafety)

[koivunej]

also adding the deny into .neon_clippy_args

[github-actions]

2358 tests run: 2243 passed, 0 failed, 115 skipped (full report)

---

Flaky tests (2)

Postgres 16

• test_emergency_mode: debug

Postgres 14

• test_restarts_frequent_checkpoints: release

Code coverage (full report)

• functions: 54.6% (8877 of 16271 functions)
• lines: 81.7% (51183 of 62642 lines)

---

_{The comment gets automatically updated with the latest test results
6ca3c27 at 2023-11-07T10:19:41.395Z :recycle:}

[koivunej]

I cannot find a way to add these to .neon_clippy_args while at the same time allowing them in postgres_ffi.