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.

examples/design-flaw.myc

@@ -0,0 +1,7 @@
+
+(do
+  (do
+    (def x "hh")
+  )
+  (print x)
+)

examples/soa_series.myc

@@ -1,4 +1,4 @@
-;; Output: 228
+;; Output: 26.7
 ;; Benchmark: 1.4us
 ;; Benchmark-Repeat: 1406
 (do

src/ast/compiler/analyzer.rs

@@ -116,7 +116,6 @@ impl<'a> Analyzer<'a> {
                 captured_by,
             } => {
                 let val_m = self.visit(value.clone());
-                let p = val_m.ty.purity;
                 (
                     BoundKind::Define {
                         name: name.clone(),
@@ -125,7 +124,7 @@ impl<'a> Analyzer<'a> {
                         value: Rc::new(val_m),
                         captured_by: captured_by.clone(),
                     },
-                    p,
+                    Purity::Impure,
                 )
             }
 

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, .. } => {