scaffolding for borrowck on MIR.

emit (via debug!) scary message from `fn borrowck_mir` until basic
prototype is in place.

Gather children of move paths and set their kill bits in
dataflow. (Each node has a link to the child that is first among its
siblings.)

Hooked in libgraphviz based rendering, including of borrowck dataflow
state.

doing this well required some refactoring of the code, so I cleaned it
up more generally (adding comments to explain what its trying to do
and how it is doing it).

Update: this newer version addresses most review comments (at least
the ones that were largely mechanical changes), but I left the more
interesting revisions to separate followup commits (in this same PR).

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