Make WalkOrder and WalkResult public (#753)

Hi, 

this PR fixes a minor visibility issue; the `WalkOrder` and `WalkResult`
enums are used in the `OperationLike::walk` method. However, we cannot
use that method from our own crates since those two enums are not public
and thus we cannot construct them.

Author: 0xddom

melior/src/ir/operation.rs

@@ -7,7 +7,7 @@ mod result;
 
 pub use self::{
     builder::OperationBuilder,
-    operation_like::{OperationLike, OperationMutLike},
+    operation_like::{OperationLike, OperationMutLike, WalkOrder, WalkResult},
     printing_flags::OperationPrintingFlags,
     result::OperationResult,
 };