Changes since 0.1.5:
- The `c_avx512` feature has been replaced by the `c` feature. In
  addition to providing AVX-512 support, `c` also provides optimized
  assembly implementations. These assembly implementations perform
  better, perform more consistently across compilers, and compile more
  quickly. As before, `c` is off by default, but the `b3sum` binary
  crate activates it by default.
- The `rayon` feature no longer affects the entire API. Instead, it
  provides the `join::RayonJoin` type for use with
  `Hasher::update_with_join`, so that the caller can control when
  multi-threading happens. Standalone API functions like `hash` are
  always single-threaded now.