Implement `if`/`unless` as an instruction

[macournoyer]

Introducing an OP_BRANCHUNLESS <offset> instruction (inspired by Ruby).

Pseudo bytecode:

<condition>         <= evaled condition is on stack
OP_BRANCHUNLESS  2  <= jump +2 bytes unless stack_pop
<2 bytes of body>
<end if>

To keep things simple at first, we could compile only the if/unless without any else branches.

This would be the first jump type instruction. if is the simplest one, so it's a good starting point. After this, porting for, case, and, or to instructions will be possible.

However, I think this is where the const_ptr pointer breaks. Any instruction using the const_ptr inside a loop/jump will cause it to be out of sync w/ ip. This should be fixed first.

[dylanahsmith]

However, I think this is where the const_ptr pointer breaks. Any instruction using the const_ptr inside a loop/jump will cause it to be out of sync w/ ip. This should be fixed first.

I would say the benefits of it start to break, since it would essentially rely on fat pointers, which include an offset for both the ip and const_ptr. I agree that we should move away from the constant pointer before implementing the control flow tags.

[dylanahsmith]

Added issue #84 for migrating away from using a moving const_ptr.