Add HeapView functionality
This pulls parts of Joachim Breitner's ghc-heap-view library inside GHC.
The bits added are the C hooks into the RTS and a basic Haskell wrapper
to these C hooks. The main reason for these to be added to GHC proper
is that the code needs to be kept in sync with the closure types
defined by the RTS. It is expected that the version of HeapView shipped
with GHC will always work with that version of GHC and that extra
functionality can be layered on top with a library like ghc-heap-view
distributed via Hackage.
Test Plan: validate
Reviewers: simonmar, hvr, nomeata, austin, Phyx, bgamari, erikd
Reviewed By: bgamari
Subscribers: carter, patrickdoc, tmcgilchrist, rwbarton, thomie
Differential Revision: https://phabricator.haskell.org/D3055