deduplicate a lot of code

compiler/rustc_const_eval/src/interpret/mod.rs

@@ -14,7 +14,7 @@ mod terminator;
 mod traits;
 mod util;
 mod validity;
-pub(crate) mod visitor;
+mod visitor;
 
 pub use rustc_middle::mir::interpret::*; // have all the `interpret` symbols in one place: here
 
@@ -27,7 +27,7 @@ pub use self::memory::{AllocCheck, AllocRef, AllocRefMut, FnVal, Memory, MemoryK
 pub use self::operand::{ImmTy, Immediate, OpTy, Operand};
 pub use self::place::{MPlaceTy, MemPlace, MemPlaceMeta, Place, PlaceTy};
 pub use self::validity::{CtfeValidationMode, RefTracking};
-pub use self::visitor::{MutValueVisitor, ValueVisitor};
+pub use self::visitor::{MutValueVisitor, Value, ValueVisitor};
 
 crate use self::intrinsics::eval_nullary_intrinsic;
 use eval_context::{from_known_layout, mir_assign_valid_types};