Get parametric rewriters working again

This is critical, but not a demo blocker for the Diff module.

This seems to be working now for parameters mapped to paths (as long as they're explicitly mapped in the arguments array of the D7 route). If that was Greek to you, don't worry: it's a physical mental note for me.

You can verify this by converting the Diff module, then looking at the methods in the default controller. With the exception of diffsOverview() -- which doesn't explicitly map its parameter to the path, and is therefore skipped due to the ambiguity (we COULD account for that edge case, but why bother...it's an edge case) -- they'll have the proper rewrites applied.