Remove extern crate rustc_middle from rustc_const_eval.

This requires exporting the interpreter macros so they can be used with
`use crate::interpret::*`.

compiler/rustc_const_eval/src/interpret/step.rs

@@ -7,6 +7,7 @@ use either::Either;
 use rustc_index::IndexSlice;
 use rustc_middle::mir;
 use rustc_middle::ty::layout::LayoutOf;
+use rustc_middle::{bug, span_bug};
 use rustc_target::abi::{FieldIdx, FIRST_VARIANT};
 
 use super::{