Implement CheckedNumCast trait for integer types.
Implement mut_chunk_iter for mutable vectors.
Digest: Add the block_size() method to the Digest trait.
Digest: Add the output_bytes() method to the Digest trait.
Crypto: Implement fixed_time_eq function.

This function compares two vectors using a fixed number of operations. This
method should be used wherever an adversary might time how long a comparison
takes and derive useful information from that.
Crypto: Implement the read_u32_le and read_u32_be functions.
Hmac: Implement the Mac trait and the Hmac function.
PBKDF2: Implement the PBKDF2 Key Derivation function.
Scrypt: Implement the Scrypt Key Derivation function.