A call to gen_tyvarify tm renames all of the type variables in term
tm to fresh replacements (generated with gen_tyvar).
FAILURE
Never fails.
EXAMPLE
> show_types := true;
> gen_tyvarify âh::tâ;
<<HOL message: inventing new type variable names: 'a>>
val it = â(h :%%gen_tyvar%%30)::(t :%%gen_tyvar%%30 list)â: term