VarInt now uses correct prefix for 64-bit

As per the [VarInt implementation in bitcoinj](https://code.google.com/p/bitcoinj/source/browse/core/src/main/java/com/google/bitcoin/core/VarInt.java); the current `253` prefix is blatantly wrong, and would be perceived by any implementation as a 16-bit variable integer.

Thankfully, this is unlikely to be a major problem in the wild.

Please don't merge until tests are committed.