interactivity fix for online check syntax

commit e503850f21aab20234f580c9e14c7e3764383e80 broke drracket's
interactivity (for some files it could take 2 seconds to do
that one line)

This changes the bindings-table so that it maps to sets instead of
lists. Now, instead of mutating all entries in the table right after
collecting everything, just leave them as sets until we need the info
and just sort a single entry, when it is needed