Milestone: Fibonacci function

[sunfishcode]

Recursive Fibonacci is a nice next milestone because it needs some control flow operators and calls, which should really help establish the shape of the backend.

The steps are:

• Implement if+else+end.
  • We'll need a stack for tracking nested control flow constructs. To start with there's only one kind of control-flow stack entry, for if/else/end, but eventually we'll have more. It can hold DynamicLables to keep track of the labels for branching.
  • The result value of an if can be carried by the stack just like normal operator results. (For now; later with on-the-fly register allocation it can be more sophisticated.)
• Implement calls
  • This is a good time to generalize the handling of function signatures. I think we can just collect all the FuncTypes from the type section into a Vec and pass that around for now.
  • Optionally, to keep things simple for this step, we could limit support to just 6 integer arguments, to avoid having to deal with stack arguments just yet.

I've left out a lot of the low-level details here; feel free to ask for more detail!

[pepyakin]

I will work on that!