Unfold [transitivity] and [symmetry] in the corresponding tactics

`Syntax error: 'rec' expected after 'let' (in [tactic:binder_tactic]).`   Ltac doesn't have a destructuring `let`.  But perhaps this would be more readable if I replaced the three lets with a single `lazymatch goal with`?  (`lazymatch`, like using `let` will give sane error messages rather than `no matching goal`.  Though, come to think of it, they might have fixed that in trunk, so that `match` also doesn't give the `no matching goal` error.)