Implicit type conversion

[wpmed92]

During the analyzer phase the AST should be rewritten so that types match correctly. The rewriting involves inserting ConstructorExpressions with the appropriate type. The codegen part of type conversion is already done.
A prerequisite for this is to modify the ASTVisitor and the AST, since the current raw pointer based approach is not right. We need a reference to the unique_ptr in the visitor methods, not a raw pointer through .get().

The implicit conversion rules can be found in the specs, in section "4.1.10. Implicit Conversions".