Implement RTTI and the CASE statement over types

[vtereshkov]

Comparing to Go, the current implementation of interfaces in XD Pascal is limited, since it lacks 'type assertions' and 'type switches' (i.e., the case statement over types, in Pascal terms). This omission, for example, makes empty interfaces meaningless, while they are widely used in Go, even for implementing Println.

Type switches require run-time type information (RTTI).