make digest an optional dependency once "weak dependencies" are available

[oconnor663]

Tracking issue: rust-lang/cargo#8832

The problem is that we have this line in Cargo.toml:

std = ["digest/std"]

That is, if you enable the std feature of this crate, then you also need the std feature of digest. The intention is that this should only matter if digest is enabled as an optional dependency. However, what it actually means is that enabling std always enables digest. This isn't a huge deal, because digest is small. But it does mean that digest isn't properly gated behind our traits-preview feature.

[elichai]

This can be closed now that #309 removed the digest/std dependency entirely