Factor runs on AMD64

The compiler backend is pretty much done; everything except for the C library interface works. A bootstrapped AMD64 factor.image is 5.4 Mb. This is much larger than the x86 image (3.3 Mb) or the PowerPC image (3.7 Mb).

Finishing the AMD64 port took a while, for several reasons. I took a break to work on the help system, and also I spent some time doing several compiler refactorings which were not required to get the backend operational, but were good for the longer-term health of the code. Whenever I revisit old code, I invariably end up refactoring or in many cases rewriting it. I hope that Factor's implementation gets tighter and cleaner over time, and not the other way around like with most code bases.

Implementing the C library interface should be a trivial but technical task; its only a few lines of code to write, but testing and debugging is always a pain, and gdb is no use with low-level issues. Once this is done I will return to working on the help system. Finishing the help system is not a high priority for 0.80, however I do need to implement enough to make the tutorial usable again, so expect a 0.80 release around the new year.