Minimal supported Rustc version

[termoshtt]

https://github.com/rust-math/intel-mkl-src/runs/7556395253?check_suite_focus=true

Current master cannot build on rustc 1.49.0

[termoshtt]

package.rust-version in Cargo.toml requires 1.56.0
https://blog.rust-lang.org/2021/10/21/Rust-1.56.0.html#cargo-rust-version

[termoshtt]

The above compile error occurs <1.53.0 rustc. So some dependent crate uses 1.54 features.

[termoshtt]

Fixed in #74 and #73