Use BLAKE2b for hashing.

...n/java/org/briarproject/bramble/crypto/Blake2sDigest.java

.../org/briarproject/bramble/crypto/CryptoComponentImpl.java

...n/java/org/briarproject/bramble/crypto/SignatureImpl.java

.../org/briarproject/bramble/crypto/TransportCryptoImpl.java

...va/org/briarproject/bramble/crypto/Blake2bDigestTest.java

...rproject/bramble/crypto/EllipticCurvePerformanceTest.java

...st/java/org/briarproject/bramble/crypto/PseudoRandom.java