On the performance of building D programs

[Dicebot]

I was very unpleasantly surprised to know from that thread about 
dmd doing everything but object file creation during rdmd 
dependency tracking step. That feels very inefficient and 
probably only reason this has not caught any attention is 
generally fast dmd compilation speed. This case begs for better 
integration between dmd and rdmd or even moving some 
functionality of rdmd to front-end.