sizedness of const parameter types

[BoxyUwU]

currently there is no restriction on const parameter types having to be sized, although it is impossible to provide any arguments to a parameter such as const N: [u8] because type checking { [1; 2] as [u8] } will fail as [u8] is not Sized. We should figure out if we want to find some way to allow !Sized const generics to work or if we want to explicitly forbid them (although if we do we should not add Sized as a supertrait of ConstParamTy as that would make it impossible to ever allow !Sized const parameter types)