Fix performance

[hellerve]

Let's face it: zepto is slooooooow. We should fix the performance as fast as possible. There are quite a few ideas I have been pondering lately:

• Rewriting the VM to a register-based byte code machine. That's relatively low-hanging fruit.
• Restart working on the nanopass compiler. That will likely take a lot of time and resources.

Both of these are valid options. Eventually, probably both will be implemented. For now, this issue exists to keep track of the progress in making zepto perform.

[hellerve]

It might be of interest for this ticket that the Haskell package llvm-general does not currently support recent versions of LLVM or GHC (see this issue). This can be resolved, but I think it is best to wait for them to publish a fix to Hackage before writing the zepto bindings around LLVM for the nanopass compiler.