owlisp does now compile to SECD virtual machine code

owlisp now has an internal SECD virtual machine and compiles expressions to byte-code that this VM understands (see README).

I have implemented a VM because I think it might be a nice abstraction layer between the Lisp code and the resulting LLVM-IR and/or Parrot VM IR.

It should help separating some complex processes from each other, like analysis of the source code & transition from dynamically typed language to statically typed LLVM-IR.

It may later also help to target different architectures (like for example Parrot VM vs. LLVM-IR), because the internal virtual machine should remain unchanged.

However, I am not yet sure if SECD is the VM that I finally want, because it is stack-based, while LLVM is register-based. If it turns out to be too complicated to translate from one to the other, I might as well change the SECD-VM to some other form of VM, maybe a custom one.