Make EvalContext::step public again

Fixes rust-lang#55061

Author: bjorn3

src/librustc_mir/interpret/step.rs

@@ -52,7 +52,7 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M>
     }
 
     /// Returns true as long as there are more things to do.
-    fn step(&mut self) -> EvalResult<'tcx, bool> {
+    pub fn step(&mut self) -> EvalResult<'tcx, bool> {
         if self.stack.is_empty() {
             return Ok(false);
         }