Add no_std (+alloc) support

[olanod]

[fdncred]

What benefits does this bring?

[olanod]

For reasons I need to use this in an embedded context with no std. I see you're hesitant to make changes to this library but I don't see this one breaking anything as the std feature is made the default and existing dependents will keep using the library in the same way.

[cptpiepmatz]

In our latest core team meeting we decided that we want this feature but we want to ensure that it doesn't break anything. So I'm going to merge it onto another branch and add CI to check for this. Thanks for the contribution!