Methods on unions

Decided

D-0078, settled by the language’s author: part of spec §18. Older text cites it as 18.78.

A union may declare methods, as a struct does; self is the union. No constructors, copy or drop, which its variants already define (18.52). Needed by #machine, whose step must belong to the machine it steps: as a free function two machines in one module would collide, with no overloading to tell them apart (§7.5)