Use uint64 for GeneralizedIndex (#13)

To support adressing any nimbus-eth2 `BeaconState` member, 32-bit wide
indices are not enough. `BeaconState` requires 5 levels of depth;
`VALIDATOR_REGISTRY_LIMIT` for the `validators` array requires 40 levels
for the contents plus 2 levels for `len` and the root. Another 4 levels
are required for `Validator`. This means that 52-bit indices are needed.
Extending to `uint64` should provide enough leeway without requiring
further implementation adjustments.

ssz_serialization/types.nim

@@ -23,9 +23,7 @@ const
   bytesPerChunk* = 32
 
 type
-  # TODO Figure out what would be the right type for this.
-  #      It probably fits in uint16 for all practical purposes.
-  GeneralizedIndex* = uint32
+  GeneralizedIndex* = uint64
 
   UintN* = SomeUnsignedInt|UInt128|UInt256
   BasicType* = bool|UintN