[Decompiler] Decompile let (#309)

* test

* fix bug

* fix tests for let

* missing formatting fix

decompiler/analysis/insert_lets.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include "decompiler/IR2/Env.h"
+#include "decompiler/Function/Function.h"
+#include "decompiler/IR2/Form.h"
+
+namespace decompiler {
+
+struct LetStats {
+  int total_vars = 0;
+  int vars_in_lets = 0;
+
+  void operator+=(const LetStats& other) {
+    total_vars += other.total_vars;
+    vars_in_lets += other.vars_in_lets;
+  }
+};
+
+LetStats insert_lets(const Function& func, Env& env, FormPool& pool, Form* top_level_form);
+
+}  // namespace decompiler