breakage from latest nightly

[dwrensha]

Due to rust-lang/rust#52597, we get a bunch of errors:

error[E0432]: unresolved import `rustc::mir::Literal`
   --> src/eval_context.rs:815:21
    |
815 |                 use rustc::mir::Literal;
    |                     ^^^^^^^^^^^^^^^^^^^ no `Literal` in `mir`

error[E0433]: failed to resolve. Could not find `Literal` in `mir`
   --> src/step.rs:203:18
    |
203 |             mir::Literal::Value { value: &ty::Const { val: ConstValue::Unevaluated(def_id, substs), .. } } => {
    |                  ^^^^^^^ Could not find `Literal` in `mir`

error[E0433]: failed to resolve. Could not find `Literal` in `mir`
   --> src/step.rs:206:18
    |
206 |             mir::Literal::Value { .. } => {}
    |                  ^^^^^^^ Could not find `Literal` in `mir`

error[E0433]: failed to resolve. Could not find `Literal` in `mir`
   --> src/step.rs:207:18
    |
207 |             mir::Literal::Promoted { index } => {
    |                  ^^^^^^^ Could not find `Literal` in `mir`

error: aborting due to 4 previous errors