fix clippy warnings

[haryu703]

When I used gf macro, I encountered following warnings in generated code.

• https://rust-lang.github.io/rust-clippy/master/index.html#assign_op_pattern
• https://rust-lang.github.io/rust-clippy/master/index.html#manual_strip
• https://rust-lang.github.io/rust-clippy/master/index.html#manual_bits

This PR fixes these warnings.

Test:

$ cargo clippy --all-features
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s

Note:
This PR may bump MSRV of this crate.

[geky]

Hi @haryu703, thanks for creating this. Sorry that I haven't been able to review the changes.

[haryu703]

@geky Thank you for responding. I updated the PR to fix new clippy warnings.
https://rust-lang.github.io/rust-clippy/master/index.html#empty_line_after_doc_comments
https://rust-lang.github.io/rust-clippy/master/index.html#suspicious_doc_comments

Following is the output of cargo clippy after the changes:

$ cargo +stable clippy --all-features
warning: unexpected `cfg` condition value: `8`
   --> src/p.rs:232:1
    |
232 | #[p(u=usize)] pub type psize;
    | ^^^^^^^^^^^^^
    |
    = note: expected values for `target_pointer_width` are: `16`, `32`, and `64`
    = note: using a cfg inside a attribute macro will use the cfgs from the destination crate and not the ones from the defining crate
    = help: try referring to `p` crate for guidance on how handle this unexpected cfg
    = help: the attribute macro `p` may come from an old version of the `gf256_macros` crate, try updating your dependency with `cargo update -p gf256_macros`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
    = note: `#[warn(unexpected_cfgs)]` on by default
    = note: this warning originates in the attribute macro `p` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: unexpected `cfg` condition value: `128`
   --> src/p.rs:232:1
    |
232 | #[p(u=usize)] pub type psize;
    | ^^^^^^^^^^^^^
    |
    = note: expected values for `target_pointer_width` are: `16`, `32`, and `64`
    = note: using a cfg inside a attribute macro will use the cfgs from the destination crate and not the ones from the defining crate
    = help: try referring to `p` crate for guidance on how handle this unexpected cfg
    = help: the attribute macro `p` may come from an old version of the `gf256_macros` crate, try updating your dependency with `cargo update -p gf256_macros`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
    = note: this warning originates in the attribute macro `p` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: `gf256` (lib) generated 2 warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.07s

2 warnings are related to this code. Could I remove #[cfg(target_pointer_width = "8")] and #[cfg(target_pointer_width = "128")]?

[geky]

I was curious about this. This seems to have broken even outside of clippy, which is a bit annoying given that the code hasn't changed.

And doesn't that warning seem a bit... naive? Given that 128-bit hardware already exists?

At least it's fair to say 8-bit processors will never exist.

Feel free to add #[allow(unexpected_cfgs)] or delete them. They shouldn't have an effect unless Rust adopts a 128-bit arch, and I don't have a strong opinion.

[haryu703]

This warning was introduced in Rust 1.80.0 and expanded in 1.84.0. Expected values are automatically set.

If 8-bit/128-bit psize is really needed, is it possible to reimplement psize using p macro? If so, I would remove them.

rust-lang/rust#65473 may be related to this.