Unified the two versions of PLEXILScript events into one, moved stuff

Now there is a single "AST" (not much of one, frankly) that represents
script events, and a visitor pattern for doing stuff with them. This
removes the weird distinction between events being processed at
translation time and at run time. 

Also took the liberty of moving a few things around in packages a little
bit. Script stuff has its own package now.