Missing #[feature(generic_const_exprs)] in calling crate causes ICE due to rustc bug

[Tehforsch]

Before, this would simply result in a compiler error saying that the feature is missing.
Currently, it causes an ICE. See this rustc issue: rust-lang/rust#133199

I thought that I might be able to throw an explicit error message from diman if the feature is not active, but this unfortunately not possible (since there is no #[cfg(...)] equivalent for compiler features). This means that the only thing to do right now is for the bug to get fixed.