Define custom [transitivity] and [symmetry].

Now that tactics like [symmetry] are defined in Coq-land in the stdlib,
thanks to Pierre-Marie Pédrot, we can define our own versions that,
after unfolding, result in proofs using things like [concat].
Remove many [path_via] and [apply symmetry]

Now [symmetry] and [transitivity] work.  Should we remove remaining uses
of [path_via], and replace it with its body, [transitivity foo; auto
with path_hints]?
Remove [path_via]

Replace remaining uses of [path_via mid] with [transitivity mid; auto
with path_hints].
Merge pull request #451 from JasonGross/transitivity

Transitivity