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