commit e503850f21 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
being edited in DrRacket (via places)
Added an API to let tools have access to that
information (and compute more stuff)
Used that to make an online version of Check Syntax
which led to a separately callable Check Syntax API.
