DT type property, File class bug

Added the .type() method to data types (objects can re-write it after
creation, not before).
Found the heisenbug that was making source code 'injections' after the
code body. It turned out that the file class could only read files up to
511 bytes.
This should be fixed, but for now the Interpreter::execute function just
reads the file differently. The file class writes work fine.