Revert "Revert "Remove support for SHA-1.""

This reverts commit db0c6cb, i.e.
it removes SHA-1 support again.

0.17.0: Use *ring* 0.17.0, untrusted 0.5.1, base64 0.6.0.

0.16.0.

Take std::time::SystemTime by value (copy) in Time::try_from.

Add src/calendar.rs to Cargo.toml; clean up duplicate entries.

0.13.0.

0.12.1: Update to *ring* 0.9.4.

0.12.1: Update to *ring* 0.9.4.

0.11.0: Require *ring* 0.8 and untrusted 0.5.

0.10.2.