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

...olable proof terms. As discussed in #454.
Quoting [the reference manual](http://coq.inria.fr/refman/Reference-Manual010.html#hevea_tactic135):
> **Variants:**
>
> 1. `simpl` _term_
>
>   This applies `simpl` only to the occurrences of _term_ in the current goal.

It looks like I might have been wrong about `simpl -[foo]`, which seems to only be for `cbv`, `lazy`, `compute`, etc (according to the reference manual); it unfolds everything except for `foo`.