cargo 0.70 transition

this will be quite involved, since it entails packaging a lot of RustCrypto crates which were not needed before.

0.70 is not the latest version, but 0.71 additionally includes a libgit2 transition and depending on the whole gix/gitoxide ecosystem, so let's take it one step at a time.

I'll update this once a list of crates and open TODOs is available, currently still slogging through the dep tree (~50 new or updated crates, and still growing)