compile a nim single file to a single c file that only depends standard c header and lib

[PMunch]

This is surprisingly hard to achieve. But even if you could do it you probably 
wouldn't get your assignment approved. The point of only allowing C is to teach 
you about pointers, manual memory management, and all the other low-level 
things C entails. Nim is great, but for teaching these concepts it isn't the 
right tool. However once you know them some things in Nim might be easier to 
reason about, so learning C is probably a good idea either way.