ConsumeInt64 and ConsumeWord64 are defined only on 32bit systems

Which makes writing portable code unnecessarily inconvenient.

Instead of 

```
decodeInt64 =
#if defined(ARCH_64bit)
  Decoder (\k -> return (ConsumeInt (\n# -> k (toInt64 n#))))
#else
  Decoder (\k -> return (ConsumeInt64 (\n64# -> k (toInt64 n64#))))
#endif
```

we could have just

```
decodeInt64 =
  Decoder (\k -> return (ConsumeInt64 (\n64# -> k (toInt64 n64#))))
```
