algorithms.c: Add basic support for Keccak[1600, n].

algorithms.c: Add basic support for Keccak[1600, n].

This takes the form of a simple object which encapsulates the
Keccak[1600, n] state and allows mix and extract operations (which
correspond to the I/O portions of absorb/squeeze and duplexing) and
step, which actually invokes the permutation to advance the state.

None of this keeps track of rate or capacity limits beyond the obvious
memory-safety checks, so you can really screw yourself if you're not
careful.