Features should be additive, don't use nostd feature, but use an std one instead

[kevinlewi]

Based on https://doc.rust-lang.org/cargo/reference/features.html?highlight=additive#feature-unification

We should replace our nostd feature to instead have an std feature that enables std.

[kevinlewi]

One example of where this should be addressed: https://github.com/facebook/akd/blob/main/akd_core/src/lib.rs#L182