Fix type equality checks and improve unmarshaling

[jazg]

Previously the type comparison inside the List constructor was just checking if the two Type interfaces were equal. This was unreliable (e.g. when comparing two typeStruct types), and as a result the Type interface has been updated to contain an Equals function. Additionally, concrete unmarshalers have been implemented for the types to ensure the data being unmarshaled is actually valid.

[loongy]

Solid implementation all round; and it is great to see some new tests added to the test suite. I have approved, but, I have also left some comments for dramatically improving equality checking performance (from O(n) to O(1)).