Split TezosWord64 into distinct types.

The TezosWord64 type was used for a long time to represent all cases where an integral type greater than 32 bits was encoded in JSON. However, it is clear from the Tezos developer documentation that many of these cases are actually supposed to be arbitrary size integers. This caused real issues in projects depending on this library.

Therefore we remove the old type entirely in favor of a system where we distinguish the large integral types in Tezos using type synonyms for the corresponding Haskell types under a StringEncode newtype wrapper. The Counter type synonym for TezosPositiveBigNum is also introduced for improved clarity.