Using fibers

[solidstate1991]

I'm porting LZHAM to D, and the original used some very unusual 
approach for coroutines:
  - the whole thing is running inside of a single switch-case 
block created by C++ macros
  - the function saves some local values
  - a macro sets the state variable to the current line number, 
returns the status (eg. if compression error happened or the 
decompressor needs more data), then the function can be continued 
from a new case block labelled by __LINE__

I started to switch out error codes with exceptions, at least 
they also provide more information besides what happened, but 
don't know what should the fate of the other status codes be. I 
probably making them derived from the Throwable class to 
distinguish them from the error codes, or put delegates in place 
of them. I hope the Fiber class will eliminate the ugliness of 
the macro-generated switch-case block, and make the code easier 
to read and debug.