Add MSRV 1.60.0

[tkaitchuck]

Adds a MSRV and an github actions test to prevent regression.
Fixes #179

[polarathene]

Hope the feedback helps 😁

Here's the TL;DR: summary:

• Your CI change with -Z msrv-policy probably isn't useful if you're not testing the Cargo.lock on the rust-version toolchain declared in Cargo.toml.
• Not sure why you're raising the minimum versions in Cargo.toml for deps, usually you don't need to do that unless there's good reason to.
• rust-version = "1.60.0" in Cargo.toml is good 👍 It's usefulness is more to do with when you need to raise it for ahash in a future release.
  • CI may be able to leverage it with -Z msrv-policy, but having the toolchain resolve a fresh Cargo.lock also communicates when that nightly feature resolves deps differently than the intended MSRV toolchain would. Until the feature is stabilized at least. Both approaches can fail, just differently.

[polarathene]

Raising minimum version in semver is not necessary unless your crate fails to build / support lower versions.

There is -Z direct-minimal-versions which (unlike -Z minimal-versions) will resolve only the lowest bound of your semver range for your dependencies (-Z minimal-versions does this for every crate in Cargo.lock which is more likely to fail). Using this would then verify a build compatibility at your lower bound, while the implicit dependencies would resolve their semver as per usual.

Raising the minimum version adds a constraint on the semver if other crates have an overlapping range, narrowing what is accepted AFAIK.

If your declared rust-version builds / resolves correctly, then there is no need to raise the minimum? It does not benefit your crate when published AFAIK, semver will be resolved when ahash is pulled in and typically use the latest release for the dependency unless Cargo.lock for example manipulates that like with the mentioned -Z nightly options.

[polarathene]

This is good 👍