Add a simple (and initially unoptimized) Keccak/SHA3 implementation
based on https://github.com/mjosaarinen/tiny_sha3/blob/master/sha3.c, as
one will be needed for TORv3 support (the conversion from BIP155
encoding to .onion notation uses a SHA3-based checksum). In follow-up
commits, a benchmark is added, and the Keccakf function is unrolled for
a (for me) 4.9x speedup.

Test vectors are taken from
https://csrc.nist.gov/projects/cryptographic-algorithm-validation-program/secure-hashing#sha3vsha3vss.

ninja check
ninja bench-bitcoin