Introduce `feature(generic_const_parameter_types)`
Allows to define const generic parameters whose type depends on other generic parameters, e.g. `Foo<const N: usize, const ARR: [u8; N]>;`
Wasn't going to implement for this for a while until we could implement it with `bad_inference.rs` resolved but apparently the project simd folks would like to be able to use this for some intrinsics and the inference issue isn't really a huge problem there aiui. (cc ``@workingjubilee`` )
415b207b7f