Add a default virtual destructor to `Monomorphiser` (ref #236)
Add a default virtual destructor to `Monomorphiser` (ref #236)

Can compile it with `-Werror=delete-abstract-non-virtual-dtor`

thepowersgang/mrustc,thepowersgang/mrustc,thepowersgang/mrustc,thepowersgang/mrustc,thepowersgang/mrustc