[Inspector] Reimplement point retention between inspector screen changes

[alexander-yakushev]

Two changes here:

1. I extracted the mechanism of binding a specific REPL connection to an inspector buffer into a separate variable in cider-connection.el and taught (cider-current-repl) to recognize it. In the future, other ancillary buffers (e.g. *cider-error*) will use this variable too instead of re-implementing this locally.
2. The current approach to remembering point location between screen changes in the inspector is not realiable. For example, it uses a single stack when navigating to prev/next page where ideally it should be a stack of stacks. As a result, the point jumps quite chaotically when using the inspector. I simplified the approach – now, there is only one stack for inspector push/pop operations. For other operations (prev/next page, siblings, non-updating operations), the point is kept in the same place where it is now. In operations that completely re-render the inspector, the point is "reset" to the first inspectable object.

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• You've updated the changelog (if adding/changing user-visible functionality)

[bbatsov]

If this REPL/connection never changes I'm guessing that the word "current" is redundant here.