Support while.exec.<proc> to ease loop maitenance

A common while pattern is to write:

```

while.true
...

end
```

This causes a bit of duplication, since the same test has to be replicated prior to the while, and as the last thing in the loop. This proposes extending the `while.true` syntax to also support `while.exec.`. The syntax would expand to the pattern above.

Example usage:

```
proc.is_not_zero
dup.0 neq.0
end

begin
push.5
while.exec.is_not_zero
sub.1
end
end
```