Use LLVM

[girving]

A few questions here are

1. Work out the data layout, and figure out how much we have to be "inside" LLVM to access data in the LLVM machine. In particular, interpreted and compiled code should operate on values stored in the same common format, so that we can freely mix the two. I don't understand LLVM well enough yet to understand how that kind of thing works (it may be simple or hard).
2. Garbage collection. For better or worse, LLVM doesn't have built in garbage collectors. Hopefully we can use someone else's LLVM garbage collector, but I haven't look at the state of such things. We should also see what HLVM is doing.
3. Bootstrapping and self-compilation. Duck code is going to be constantly recompiling itself, which will be generating new LLVM code. This will require some thought and a lot of LLVM understanding.