Deprecate `maybe_field()` setters in favor of `field_if_some()`.

Some builder convenience setters where added to work with Optional values. This is the case for the Pad builder setter maybe_name() which was introduced as part of !1255 (merged).

Then more convenience setters were discussed and it was decided to use field_if_some() instead of maybe_field(). Existing maybe_field()s were kept for backward compatibility. They can now be marked as deprecated referring to the matching field_if_some().