Merge branch 'update_visitor'

This brings a new update_visitor that makes it easier to traverse the
parse tree and modify parts of it as necessary.  I wrote this as part of
my in-progress work to allow @cast() expansion, but I was able to apply
it to the dwarf/etc. target variable expanders and to the optimization
stages.  I think the resulting code is more predictable and easier to
follow...

Conflicts:
	ChangeLog (bumped my commit dates to push dates...)

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