Add finish_into to write the hash to a user provided buffer.

[calmofthestorm]

• This avoids needing to allocate a vector for every hash on Windows and
  Linux. The CommonCrypto API will work but still allocates a vector.

• The buffer must match the digest size exactly or panic. An undersized
  buffer would violate Rust safety guarantees, but an oversized buffer
  could also be problematic if, e.g., the user passed a 32 byte buffer
  into a 20 byte digest without zeroing it first.

• The main benefit is performance when hashing enormous numbers of
  values, but this can also be more ergonomic in certain cases.
  Consider:

struct Oid {
  sha1: [u8 ; 20],
}

fn one() {
  let mut oid = Oid::default();
  // ...
  hasher.finalize_into(&mut oid.sha1);
}

// vs

fn two() {
  let mut oid = Oid::default();
  // ...
  let digest = hasher.finalize();
  oid.sha1.copy_from_slice(&digest);
}

• Also fix a bug in the helpers in lib.rs which should call flush
  after writing to the Hasher.