Fiber implementation

[mogu]

I made a fiber implementation just now 
([nimfiber](https://github.com/mogud/nimfiber)). And it was almost borrow from 
standard lib **coro**. But I'm a noob to nim, so don't understand some APIs 
used in coro: 
    
    
    proc GC_addStack(starts: pointer) {.cdecl, importc.}
    proc GC_removeStack(starts: pointer) {.cdecl, importc.}
    proc GC_setCurrentStack(starts, pos: pointer) {.cdecl, importc.}
    
    proc getFrame*(): PFrame {.compilerRtl, inl.}
    proc setFrame*(s: PFrame) {.compilerRtl, inl.}
    
    

Please help! T.T

And another issue is that should coro be implemented like this to be more 
useful? Some high level library may heavily use fiber like goroutine/actor/rpc.