Clean up naming of VariableList/FixedVector on tree-states

[michaelsproul]

Description

At the moment tree-states has a confusing mix of type aliases for the VariableList/FixedVector types. To remedy the situation I'm proposing to instead use these names:

• VariableList/FixedVector: same as on stable, refer to types from ssz_types
• List/Vector: unmodified milhouse type names

Alternatively, we could alias both libraries with new names:

• TreeList/TreeVector: type aliases for milhouse types
• FlatList/FlatVector: type aliases for ssz_types types

However I think avoiding aliases altogether is probably clearer.

[dapplion]

• VariableList/FixedVector: same as on stable, refer to types from ssz_types
• List/Vector: unmodified milhouse type names

+1