Add gotxs/easy package for OT_ME wrappers

The `gotxs` package should only wrap `OTAPI_Wrap*` methods.

All Go methods wrapping the `OT_ME` class should go to `gotxs/easy` package

This makes it easier to see which level wrapper is actually called. Also there
might be situations where need a higher-level and a lower-level wrappers and
needed, this way both can coexist.