Merge branch '236-curve25519' into 'master'

.../org/briarproject/bramble/api/crypto/CryptoConstants.java

bramble-core/build.gradle

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

.../org/briarproject/bramble/crypto/Curve25519KeyParser.java

...org/briarproject/bramble/crypto/Curve25519PrivateKey.java

.../org/briarproject/bramble/crypto/Curve25519PublicKey.java

...g/briarproject/bramble/crypto/EllipticCurveConstants.java

...oject/bramble/crypto/EllipticCurveMultiplicationTest.java

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

...ava/org/briarproject/bramble/crypto/KeyAgreementTest.java

...riarproject/bramble/crypto/KeyEncodingAndParsingTest.java