`chain_ref` and `chain_mut_ref` were removed.  According to irc discussion, this was deliberate.  I don't mind the alpha-renames, but I'm a little hesitant to approve outright removal of methods.

(I grok that they were not used often within the rustc code base, from the number of occurrences that had to be updated in this PR; I don't know about whether they are used rarely elsewhere as well.)