Fix issue #5 : forward declaration of structures

https://github.com/jacob-carlborg/dstep/issues/5

In essence. dstep now will check if given struct declaration is also a definition and output code for forward declared structures only if no definition was later found in translation unit.

As a bonus, forward declared struct code is generated in a shorter C-like syntax:
struct Fwd {} -> struct Fwd; 
Can't find relevant stuff in dlang.org spec but at least generated code does not seem to differ.