Auto merge of #8857 - ehuss:resolver-1, r=alexcrichton

Auto merge of #8857 - ehuss:resolver-1, r=alexcrichton

Allow resolver="1" to explicitly use the old resolver behavior.

A couple people expressed that it was surprising that it was not possible to explicitly specify the old behavior, so this PR adds the ability to specify `resolver = "1"` to explicitly specify the old feature unification behavior.   This is a little more consistent with other options like `edition` which allows an explicit "2015" even though that is the default.