[plt-dev] arrows crossing selection

I've just snuck a new feature into Check Syntax. You can now tack all 
binding arrows that cross the editor's selection boundary. That is, they 
have one end inside the selection and one end outside. One use is to 
find free variables in an expression you want to lift out as a separate 
procedure.

To use it: Run Check Syntax, highlight a region of code, and right-click 
to get the Check Syntax context menu. It has an additional item labeled 
"Tack arrows crossing selection".

If people like it, I can tidy it up and add variants (eg, untack, don't 
include arrows for module imports). If people don't like it, I can take 
it out again. Feedback welcome.

Ryan