Expand mir dump in order to handle NLL pass

Extend `dump_mir` and functions it calls in order to allow callers to
add custom information. We do this by adding an enum `PassWhere` and
an extra argument of type `FnMut(PassWhere, &mut Write) ->
io::Result<()>`.  This callback is responsible for printing the extra
information when MIR is dumped at various stages.

For the "nll" pass, use the new mechanism to dump the `Region`
information after the header, but before the control flow graph for
every function.

In the interest of keeping the output somewhat concise, implement
a custom Debug impl for `Region`

Open Questions:

    * What should we call what has been called `PassWhere` so far?

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