`core::num::bignum` could use `u64` limbs

[This line](https://github.com/rust-lang/rust/blob/4b696e6bf77b4df02634d346637bd35f2960c5f4/library/core/src/num/bignum.rs#L63) of `core::num::bignum` mentions that using `u64` limbs was blocked on `u128`. That was stabilized a while ago, so `u64` limbs can be enabled now.