Some simple improvements to MIR pretty printing

Some simple improvements to MIR pretty printing

In short, this PR changes the MIR printer so that it:

* places an empty line between the MIR for each item
* does *not* write an empty line before the first BB when there are no
  var decls
* aligns the "// Scope" comments 50 chars in (makes the output more
  readable)
* prints the scope comments as "// scope N at ..." instead of "//
  Scope(N) at ..."
* prints a prettier scope tree:
 * no more unbalanced delimiters!
 * no more "Parent" entry (these convey no useful information)
 * drop the "Scope()" and just print scope IDs
 * no braces when the scope is empty