zeroize: run clippy, clean up some docs and typos

[brxken128]

This PR is rather self explanatory. I opted to allow inline(always) as in an application there's a good chance thst these will be pretty hot paths.

I also allowed wildcard imports within the aarch64 and x86 modules, as it'd just be a pain to keep track of all of the separate types.

Additionally, I chose to keep the notice about the lack of support for clearing registers. Inline-ASM is now a thing and end users are able to use it as they wish, but it's not specific to SIMD registers (which I failed to understand at first). We could maybe remove the part about the rustc support requirement/update it a little to make it more accurate for the current state of Rust, but I'm not really sure in that regard.

[tarcieri]

I'm not sure why this PR is needed. We already run clippy against all crates as part of the workspace workflow: https://github.com/RustCrypto/utils/blob/1dcf52ec901c01f24dc5c6d4b3df06cf773b6438/.github/workflows/workspace.yml#L16

Locally clippy passes for me with Rust 1.72 installed:

$ rustc --version
rustc 1.72.0 (5680fa18f 2023-08-23)
$ cargo clippy --all-features -- -D warnings
warning: some crates are on edition 2021 which defaults to `resolver = "2"`, but virtual workspaces default to `resolver = "1"`
note: to keep the current resolver, specify `workspace.resolver = "1"` in the workspace root's manifest
note: to use the edition 2021 resolver, specify `workspace.resolver = "2"` in the workspace root's manifest
    Finished dev [optimized + debuginfo] target(s) in 0.00s

[brxken128]

I have my flycheck configured with some (probably too harsh) settings, and these are just what were picked up while I was working on other parts of the crate - feel free to close this if you though as I can see why it's probably useless (although size_of was spelt size_pf in one spot, I could probably slip that singular change into one of my other open PRs though if preferable)

[tarcieri]

I think clippy fixes should correspond to lints enabled in lib.rs, which also makes them reproducible (at least for a given version of clippy).

Since we're clean under the default configuration on the latest stable release of Rust, I'm going to close this.

[tarcieri]

Also PRs for typos welcome