Forbid reopening generic types with different type var splats

[HertzDevil]

The following (taken from specs) currently compiles:

class Foo(A); end
class Foo(*A); end

class Bar(*A); end
class Bar(A); end

class Baz(*A, B); end
class Baz(A, *B); end

This is misleading because when reopening a generic class/struct the compiler already checks the arity and the names of the formal type vars, but not the splat indices, which affect how many type vars are allowed in instantiations and which formal vars they are bound to. This patch rejects code like the above. (It remains possible to reopen a generic type with omitted type vars.)

Notice how I said "class/struct"; Crystal performs no checks on reopened generic modules, not even their arity. This patch also adds them.