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

[GHC]

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

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