Support no_std

[wangrunji0408]

Now feature alloc is available in stable Rust.
It's time to make it no_std compatible.

Similar to PR #5 .

[marcianx]

Thank you for contributing!
I'm seeing a bunch of

error[E0433]: failed to resolve: maybe a missing crate `core`?

errors when running tests via cargo test. Could you possibly resolve these?

[marcianx]

And just to emphasize, I'd love for this to be fixed without updating the tests themselves to test backward-compatibility.

[wangrunji0408]

I realized that my previous implementation was not correct. It will break on std.
So I rewrote it by referring to this guide.
And now it works well for both std and no_std with all tests unchanged.

I also added GitHub Actions to make sure everything is fine. You can check the result here.

[marcianx]

Thank, you this is excellent!

[marcianx]

Huh, rebase didn't rebase the common commit properly. I'll fix it offline and force a push. You might have trouble syncing with your branch due to merge conflicts.