The following new debug context APIs are for implementing debugger's
`next` (step over) and similar functionality.

* `rb_debug_inspector_frame_depth(dc, index)` returns `index`-th
  frame's depth.
* `rb_debug_inspector_current_depth()` returns current frame depth.

The frame depth is not related to the frame index because debug
context API skips some special frames but proposed `_depth()` APIs
returns the count of all frames (raw depth).