Forbid 'eval' to be run without context

It just doesn't make any sense, and leads to bugs in IDE mode
Fix displaying of local variables

In XML, we now correctly display type and value (which was displayed
incorrectly, because we converted everything into a string by #inspect).

In Plain text, added exception to display nil as 'nil', because by
default it is displayed as '' by #to_s method. And use #to_s for
everything else