Implement new Expression::toDt routines.
Implement new Declaration::toDt routines.
Implement new Dsymbol::toObjFile routines.
Remove old Symbol data.
Remove Obj::objmod abstraction layer.
Update d_finish_symbol (outdata) to handle new dt_t and send symbols to the back end.
Remove old dt_t functions and dmd backend wrappers.
Remove symbol.h
White space and comment updates.
Clean up StringExp::toDt implementation.