Which cleans out the pkg-mod to just the expected exports with (its
longstanding todo comment list) and thus a separation-of-concerns
and smaller mod-file sizes via the following new sub-mods:
- `._trace` for the `.pause()`/`breakpoint()`/`pdb.set_trace()`-style
APIs including all sync-caller variants.
- `._post_mortem` to contain our async `.post_mortem()` and all other
public crash handling APIs for use from sync callers.
- `._sync` for the high-level syncing helper-routines used throughout the
runtime to avoid multi-proc TTY use collisions.
And also,
- remove `hide_runtime_frames()` since moved to `.devx._frame_stack`.
16dbf8fbca