Doubling the call stack when extending it

[dearblue]

Previously it was "approximately double".

---

Simulating the current behavior and this patch's behavior, the following is shown:

• before this patch

  % ruby -e 'size = 32; limit = 512; loop { size = (size - 1) * 2; p size; break if size >= limit }'
  62
  122
  242
  482
  962

• after this patch

  % ruby -e 'size = 32; limit = 512; loop { size = size * 2; p size; break if size >= limit }'
  64
  128
  256
  512

[matz]

I am not sure about the benefit of this change. Could you explain, please?

[dearblue]

Thank you for your review.

As presented as a simulation, the current expansion is roughly twice as wide.
If exactly doubled, it would be more predictable when specifying MRB_CALL_LEVEL_MAX.

[matz]

Understood. I admit there's no clear benefit from the current behavior. Accepted.