api: fix Map key processing

[ixje]

The problems

1. The NEO Virtual Machine has a Map type which translates to a dict on the Python side. The keys are restricted to what they call PrimitiveTypes being Boolean , Integer and ByteString stack item types (respectively bool, int and str in Python).

   However, it is also possible to push bytes using the PUSHDATA opcodes which will be converted to a ByteString stack item type. Therefore we can no longer assume that when the VM returns a ByteString stack item that it is always a str.

2. int key types would incorrectly be converted to str.

The solution
This PR removes the "ByteString == str" assumption and also removes the int -> str conversion.

[coveralls]

coverage: 80.755% (+0.004%) from 80.751% when pulling e2d5ae9 on map-int-key into 4ac098c on master.