Refactor analyzer to return impurity

The `Analyzer` is now responsible for determining the purity of `Define`
bindings. Previously, this responsibility was left to the caller.

Added a new example `design-flaw.myc` to test this change.
Updated `soa_series.myc` due to the purity change.

src/ast/compiler/optimizer/utils.rs

@@ -143,7 +143,8 @@ impl UsageInfo {
                     self.collect(v);
                 }
             }
-            BoundKind::Define { value, .. } => {
+            BoundKind::Define { addr, value, .. } => {
+                self.assigned.insert(*addr);
                 self.collect(value);
             }
             BoundKind::Expansion { bound_expanded, .. } => {