Merge branch 'bc/rust-hash-cleanups'

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