Support for truncated digests was removed in Nettle 4.0. The digest
functions no longer accept the output length. Provide a full-length
buffer and copy the requested length of the digest, same as with the
other crypto providers.
For consistency and safety, change the CMC and HSH functions to accept
signed lengths and handle negative values as errors. Also, change the
input data type to void * to not require casting in the caller.
