Ensure type tracking for subtypes.

When a source type is determined to be a subtype equivalent of a target
type, then there is no need to do any remapping. However, we still need
to track that the source type should be substituted everywhere by the
target type.

Signed-off-by: Ryan Levick <ryan.levick@fermyon.com>

Author: rylev

crates/wac-types/src/aggregator.rs

@@ -131,6 +131,9 @@ impl TypeAggregator {
                     )
                     .is_ok()
                 {
+                    // Keep track that the source type should be replaced with the
+                    // target type wherever it's used.
+                    self.remapped.insert(source_kind.ty(), target_kind.ty());
                     continue;
                 }