cargo: point repository metadata to clonable URLs

[lucab]

This tweaks the repository fields in Cargo metadata in order to use the correct (i.e. git clonable) URL. The existing GitHub webUI URLs for each package have been retained and moved to homepage fields.

[lucab]

For reference, without this PR the remote fetch would fail with:

$ git clone https://github.com/RustCrypto/elliptic-curves/tree/master/bign256

Cloning into 'bign256'...
fatal: repository 'https://github.com/RustCrypto/elliptic-curves/tree/master/bign256/' not found

[lucab]

@tarcieri I think I've now completed the tour of all repos under RustCrypto and fixed all the misaligned values. Sorry for the sudden spike of PRs and thanks for promptly merging all of those!

[tarcieri]

@lucab thanks for getting them corrected! I know people are trying to write tooling to automatically verify this information so it's good to have them valid