Computed goto experiment

[rdunnington]

Now that ziglang/zig#8220 has landed in mainline zig, the next release (0.14) will have computed goto support. This will open up another potential path for structuring the interpreter core loop to avoid using tail calls, which are apparently unsupported on some architectures such as MIPS (#57). If the performance difference isn't too bad, or improves, the core loop could be switched over entirely to this mechanism.