Make primitive type bounds a typechecker error

[ReedOei]

Currently, the following code:

contract X implements int {}

gives a parser error, because int is a primitive type, which cannot be implemented. It would be ideal to make this into a typechecking error instead so the compilation process can continue.

This can be done as follows:

1. Create a new class extending GenericBound (e.g., PrimitiveBound).
2. In an AST transform, log an error every time a PrimitiveBound is encountered, and turn it into a standard Top bound.