Previous semantics were encouraged by sol2, and ThePhD admitted it was a bad idea for usertypes with lots of members. sol3 allows to assign members outside a constructor for regular usertypes, but mame can't move to sol3 because it requires C++17. Turns out sol2 also has a way to add members separately, and that is what I used. This helps immensely.
This approach required a fix to warning C5046 copied from here 5799084572
Tested current mame plugins, everything seems to work.
PS: Due to come black magic, if I use simple_usertype::set() with "manager" as well, "machine" member dies (others are fine!), so I had to leave it intact.
