program: convert value when creating rvalue

This is useful for a few cases:

- We should make sure that the value of an object is actually valid for
  its type
- When casting, the value should be converted to the new type where
  appropriate
- For the upcoming ProgramObject operators

drgn/program.py

@@ -66,6 +66,8 @@ class ProgramObject:
         self.address_ = address
         self.type_ = type
         self._real_type = type.real_type()
+        if value is not None:
+            value = self._real_type.convert(value)
         self._value = value
 
     def __dir__(self) -> Iterable[str]: