Merge new commands from ghci-ng (re #10874)wip/T10874

This was for the major part implemented by Chris Done on

 https://github.com/chrisdone/ghci-ng

and has been in use by Emacs's `haskell-mode` for about a year already.

I've squashed the commits, rebased to GHC HEAD, and cleaned up the patch.

-----

The new commands this commit adds are (description copied from [1]):

* The `:set +c` command: collect information about modules after
  they've been loaded, and remember it between loads (including failed
  ones). I recommend adding this line to your `~/.ghci` file.

* The `:type-at` command (requires `+c`): show the type at the given position in the
  module. Example:

  ``` haskell
  *X> :type-at X.hs 6 6 6 7 f
  Int -> Int
  ```

  This can be useful to get the type of a pattern variable or an
  arbitrary selected expression.

  The parameters are:

        <filename> <line> <col> <end-line> <end-col> <text>

  `text` is used for when the span is out of date, i.e. the file
  changed and the code has moved, text can be used to confirm the
  contents of the span matches, and to fallback to a general :t-like
  lookup.

* The `:loc-at` command (requires `+c`): get the location of the thing at the given
  position in the module. Example:

  ``` haskell
  *X> :loc-at X.hs 6 14 6 16 mu
  X.hs:(8,7)-(8,9)
  ```

  This is useful for goto-definition features of editors and IDEs.

  Parameters are the same as for `:type-at`.

* The `:uses` command (requires `+c`): gets all module-local uses of
  the thing at the given position in the module. Example:

  ``` haskell
  > :uses /home/chris/Projects/ghci-ng/ghc/GhciFind.hs 53 66 53 70 name
  /home/chris/Projects/ghci-ng/ghc/GhciFind.hs:(46,25)-(46,29)
  /home/chris/Projects/ghci-ng/ghc/GhciFind.hs:(47,37)-(47,41)
  /home/chris/Projects/ghci-ng/ghc/GhciFind.hs:(53,66)-(53,70)
  /home/chris/Projects/ghci-ng/ghc/GhciFind.hs:(57,62)-(57,66)
  ```

  This is useful for highlighting and navigating all uses of an
  identifier in editors and IDEs.

  Parameters are the same as for `:type-at`.

* The `:all-types` command (requires `+c`): list *all* types in the project:
  expressions, bindings top-level and local. Sort of like `:browse` on
  steroids.

  ``` haskell
  > :all-types
  ghc/GhciTypes.hs:(38,13)-(38,24): Maybe Id
  ghc/GhciTypes.hs:(45,10)-(45,29): Outputable SpanInfo
  ghc/GhciTypes.hs:(45,10)-(45,29): (Rational -> SpanInfo -> SDoc) -> Outputable SpanInfo
  ```

[1]: https://github.com/chrisdone/ghci-ng/blob/master/README.md

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