Add unwrite() to CodeWriter to help with CSVs

[mtdowling]

It's sometimes easier to deal with comma separated values by just always
throwing a ", " at the end of each element in the list, even for the
last element. The unwrite() method allows you to remove the last text
written to the CodeWriter if it matches a given substring. This
allows, for example, ", " to be "unwritten" to removing the trailing
comma without having to first check if the last text written was a
trailing comma. This is similar to Ruby's chomp() method.

Issue #, if available:

Description of changes:

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[mullermp]

LGTM. This is also useful for new lines. If we're iterating to code generate methods, we want new lines after each method unless it's the last. Ruby classes might be like

class Foo
  def meth1
  end
              <- new line
  def meth2
  end <-- no new line
end