Move Sy's data from g:sy to b:sy

While I was under the foolish impression that #88 was due to Sy placing signs using `file=` instead of `buffer=`, I refactored Sy to store its data in `b:sy` instead of `g:sy`.  Although it didn't solve the original problem, it has simplified certain aspects of the code.

I figured I'd at least toss this over the wall to see what you think.