std: Constify the bytes sent to Sha1::input

The Sha1 API takes the data to be hashed as &[u8]. This patch changes the type to &[const u8]. I'm not well-versed in the const semantics of Rust, but I imagine this is a reasonable thing to do, and this change makes the API compatible with to_bytes::IterBytes, which returns data in chunks of type &[const u8].

After making this change, 'make check' reports no failures for me.

(By the way, is there a reason for the double parentheses around the function argument lists in the Sha1 trait?)