Convert hasher to interface and remove spihash64

tests/hasher/test_hasher.h

@@ -10,6 +10,5 @@ WpTestFuncResult test_murmur3(void);
 
 // Test Sip hash implementation against the reference implementation
 WpTestFuncResult test_siphash_128(void);
-WpTestFuncResult test_siphash_64(void);
 
 #endif // !TEST_HASHER_H