Safe Transmute: Refactor error handling and Answer type

- Create `Answer` type that is not just a type alias of `Result`
- Remove a usage of `map_layouts` to make the code easier to read
- Don't hide errors related to Unknown Layout when computing transmutability

compiler/rustc_transmute/src/layout/tree.rs

@@ -188,14 +188,14 @@ pub(crate) mod rustc {
         /// The layout of the type is unspecified.
         Unspecified,
         /// This error will be surfaced elsewhere by rustc, so don't surface it.
-        Unknown,
+        UnknownLayout,
         TypeError(ErrorGuaranteed),
     }
 
     impl<'tcx> From<LayoutError<'tcx>> for Err {
         fn from(err: LayoutError<'tcx>) -> Self {
             match err {
-                LayoutError::Unknown(..) => Self::Unknown,
+                LayoutError::Unknown(..) => Self::UnknownLayout,
                 err => unimplemented!("{:?}", err),
             }
         }