version_67: Strip the producers section in --strip-producers (#1875)

WebAssembly/tool-conventions#93 has a summary of emscripten's current thinking on this. For Binaryen, we don't want to do anything to the producers section by default, but do want it to be possible to optionally remove it. To achieve that, this PR

 * creates a --strip-producers pass that removes that section.
 * creates a --strip-debug pass that removes debug info, same as the old --strip, which is still around but deprecated.

A followup in emscripten will use this pass by default.