Merge branch 'bc/rust-hash-cleanups' into jch

A few memory problems in the Rust interface to C hash functions have been corrected. The 'Clone' implementation of 'CryptoHasher' now properly initializes the context before cloning, and its 'Drop' implementation now discards the context to prevent leaks. * bc/rust-hash-cleanups: rust: discard hash context when finished hash: initialize context before cloning