fix: make Grid hashable if T is hashable

[henryiii]

I think this should work to make this hashable if the element type is hashable. Closes #43.

[becheran]

Thanks for the pull request. It seems not to work though if not using the std lib. Which is supported by this crate. See failing pipeline with --no-default-features set.

[henryiii]

I think that fixes it. I didn't have a hash impl (std::collections::hash_map::DefaultHasher) to use without std, so didn't test it without std. But it compiles without std now and works with std when I try locally without default features.