Make the Rust and OCaml name matchers consistent with each other

[sonmarcho]

The Rust and OCaml name matchers have quite a few differences: it may be good to reduce the gap.
For instance, adding the syntax impl ... for ... for the OCaml name matcher is a good idea.

[Nadrieril]

Noting here so we don't forget: we likely want impl ... for .. to only match traits while impl ... can match both traits and types.