bump msrv to 1.64.0

[tcoratger]

A bump of MSRV is done to fix:

error: package `rayon-core v1.12.0` cannot be built because it requires rustc 1.63 or newer, while the currently active rustc version is 1.56.0

rustc 1.63 has a problem with compilation that's why this is directly bumped to 1.64.0

[str4d]

MSRV 1.56 does work, if you use rayon-core 1.10. We now have Cargo.lock checked in and CI jobs to ensure that tests pass on an MSRV-compatible set of dependencies, as well as latest dependencies with stable Rust. So this breakage will no longer occur when making PRs on this repo, and this MSRV bump is not necessary.

There might be other reasons we want to bump MSRV, but those should be discussed in a separate issue.