Using the standard (type class based) transitivity tactic for more contr...

...olable proof terms. As discussed in #454.
@spitters, here are the relevant bits of my `HoTT/.git/config` file:
```
[remote "upstream"]
        url = git@github.com:HoTT/HoTT.git
        fetch = +refs/heads/*:refs/remotes/upstream/*
        fetch = +refs/pull/*/head:refs/remotes/upstream/pr/*
[branch "master"]
        remote = upstream
        merge = refs/heads/master
        pushurl = git@github.com:JasonGross/HoTT.git
[remote "origin"]
        url = git@github.com:JasonGross/HoTT.git
        fetch = +refs/heads/*:refs/remotes/origin/*
```
You will obviously want `spitters` rather than `JasonGross`, and if you are set up to use https urls, you'll probably want to keep that rather than the git:// urls.  The relevant bit is probably the `pushurl` bit of `master`.