First draft, sub-msg-spec for debugger `Lock` sys

Since it's totes possible to have a spec applied that won't permit
`str`s, might as well formalize a small msg set for subactors to request
the tree-wide TTY `Lock`.

BTW, I'm prolly not going into every single change here in this first
WIP since there's still a variety of broken stuff mostly to do with
races on the codec apply being done in a `trio.lowleve.RunVar`; it
should be re-done with a `ContextVar` such that each task does NOT
mutate the global setting..

New msg set and usage is simply:
- `LockStatus` which is the reponse msg delivered from `lock_tty_for_child()`
- `LockRelease` a one-off request msg from the subactor to drop the
  `Lock` from a `MsgStream.send()`.
- use these msgs throughout the root and sub sides of the locking
  ctx funcs: `lock_tty_for_child()` & `wait_for_parent_stdin_hijack()`

The codec is now applied in both the root and sub `Lock` request tasks:
- for root inside `lock_tty_for_child()` before the `.started()`.
- for subs, inside `wait_for_parent_stdin_hijack()` since we only want
  to affect the codec *for the locking task*.
  - (hence the need for ctx-var as mentioned above but currently this
    can cause races which will break against other app tasks competing
    for the codec setting).
- add a `apply_debug_codec()` helper for use in both cases.
- add more detailed logging to both the root and sub side of `Lock`
  requesting funcs including requiring that the sub-side task "uid" (a
  `tuple[str, int]` = (trio.Task.name, id(trio.Task)` be provided (more
  on this later).

A main issue discovered while proto-testing all this was the ability of
a sub to "double lock" (leading to self-deadlock) via an error in
`wait_for_parent_stdin_hijack()` which, for ex., can happen in debug
mode via crash handling of a `MsgTypeError` received from the root
during a codec applied msg-spec race! Originally I was attempting to
solve this by making the SIGINT override handler more resilient but this
case is somewhat impossible to detect by an external root task other
then checking for duplicate ownership via the new `subactor_task_uid`.
=> SO NOW, we always stick the current task uid in the
   `Lock._blocked: set` and raise an rte on a double request by the same
   remote task.

Included is a variety of small refinements:
- finally figured out how to mark a variety of `.__exit__()` frames with
  `pdbp.hideframe()` to actually hide them B)
- add cls methods around managing `Lock._locking_task_cs` from root only.
- re-org all the `Lock` attrs into those only used in root vs. subactors
  and proto-prep a new `DebugStatus` actor-singleton to be used in subs.
- add a `Lock.repr()` to contextually print the current conc primitives.
- rename our `Pdb`-subtype to `PdbREPL`.
- rigor out the SIGINT handler a bit, originally to try and hack-solve
  the double-lock issue mentioned above, but now just with better
  logging and logic for most (all?) possible hang cases that should be
  hang-recoverable after enough ctrl-c mashing by the user.. well
  hopefully:
  - using `Lock.repr()` for both root and sub cases.
  - lots more `log.warn()`s and handler reversions on stale lock or cs
    detection.
- factor `._pause()` impl a little better moving the actual repl entry
  to a new `_enter_repl_sync()` (originally for easier wrapping in the
  sub case with `apply_codec()`).

tractor/devx/__init__.py

@@ -27,7 +27,6 @@ from ._debug import (
     pause as pause,
     pause_from_sync as pause_from_sync,
     shield_sigint_handler as shield_sigint_handler,
-    MultiActorPdb as MultiActorPdb,
     open_crash_handler as open_crash_handler,
     maybe_open_crash_handler as maybe_open_crash_handler,
     post_mortem as post_mortem,