Adapt to portable-simd to_int -> to_simd rename, bump to version 0.6.10

[jieyouxu]

Noticed during a rustfmt subtree sync attempt rust-lang/rustfmt#6807.

Context

portable-simd renamed to_int to to_simd in rust-lang/portable-simd#478 due to a common confusion (rust-lang/portable-simd#477), so bytecount with generic-simd against later nightlies fail to build.

With this patch, bytecount with generic-simd feature successfully builds on a x86_64-unknown-linux-gnu host against latest nightly rustc 1.95.0-nightly (c04308580 2026-02-18).

cargo check --features=generic-simd --all-targets

Review notes

• Split into two commits. Commit 1 does a minimal change to fix the renamed method (to_simd). Commit 2 bumps the version to 0.6.10 and updates the version referenced in the README.
• Intentionally did not run rustfmt, since it seems that the code behind generic-simd were not previously ran through rustfmt.
• This will need a new point release. I don't know how the CI works for this particular repo. I only checked locally that it check-builds.

[jieyouxu]

Hm, I think this part in the README

Versions from 0.6.0 require Rust 1.32.0 or later.

Is also not quite accurate (for the last generic-simd fix as well as this one), since when the portable-simd stuff gets renamed it's not backwards-compatible with older Rust toolchains. This particular PR would raise the "MSRV" of the generic-simd to the nightly toolchain that contains rust-lang/portable-simd#478.

Let me know if you want me to also update that particular description.