[GHC] #10874: Implement `:type-at`, `:all-types`, `:loc-at` in GHCi

[GHC]

#10874: Implement `:type-at`, `:all-types`, `:loc-at` in GHCi
-------------------------------------+-------------------------------------
        Reporter:  hvr               |                   Owner:
            Type:  feature request   |                  Status:  patch
        Priority:  normal            |               Milestone:  8.0.1
       Component:  GHCi              |                 Version:  7.10.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:  Phab:D1240
-------------------------------------+-------------------------------------
Changes (by hvr):

 * status:  new => patch
 * differential:   => Phab:D1240


Old description:

> We missed the opportunity to merge this for GHC 7.10 but I hope this can
> make it for GHC 8.0
>
> See https://github.com/chrisdone/ghci-ng/blob/master/README.md for a
> description of the new GHCi commands
>
> I've started rebasing the changes from Chris' ghci-ng repo against GHC
> HEAD and pushed the work-in-progress state as branch `wip/T10874`.
>
> One blocker right now is the use of SYB to traverse the AST which I don't
> want to drag into GHC as a dependency. So one thing that needs to be done
> is rewrite those 20 lines of code without relying on SYB.
>
> Other than that some minor polishing needs to be done.

New description:

 We missed the opportunity to merge this for GHC 7.10 but I hope this can
 make it for GHC 8.0

 See https://github.com/chrisdone/ghci-ng/blob/master/README.md for a
 description of the new GHCi commands

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10874#comment:1>
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