Moved class visitor to its own file.

Renamed SigVisitor to better indicate what it is used for.
Added 'instanceof' bound type to Rascal.
'Finished' @types and @typeDependencies. Some data is not 100% correct; no distinction between interface/class at some points.
Some TODOs added to look at at a later date.

Bit of cleanup here and there but can probably be improved further.