We should deprecate C-style declarations

[Don]

Before TDPL is published, I think we should finally get rid of C-style 
declarations. Supporting two styles of declarations just causes 
confusion, and AFAIK it doesn't have any benefits -- for example, it 
doesn't significantly help in porting code from C. D declarations are 
just better. And there are bugs such as this one, which seem to require 
abandoning C declarations:

bug #2392 Parsing ambiguity between function pointer declaration and 
function call

At the very least, can we make sure that TDPL doesn't commit us to 
retaining them forever?