feat: Switch to stable Rust and update rust-seq2kminmers dependency

[MrTomRod]

This PR updates mapquik to use stable Rust and switches the rust-seq2kminmers dependency to the MrTomRod/rust-seq2kminmers fork (see rchikhi/rust-seq2kminmers#5), which supports stable Rust.

Users can now build and run mapquik with just cargo build --release.

Key Changes:

• Removed nightly Rust requirement: mapquik now builds and runs on the stable toolchain.
• Updated rust-seq2kminmers dependency to use the my fork, which replaces unstable intrinsics with stable equivalents.
• Bumped version to 0.1.1 to reflect these changes.
• Updated README: Removed instructions for nightly Rust, as it is no longer required.

Why?

Without these changes, I was unable to build mapquik.

I did not do enough testing. Please perform some sanity checks before merging.