Control flow evaluator; Support for block-level let/const variables

tests/compiler/scoped.ts

@@ -0,0 +1,14 @@
+var aGlobal: i32 = 1;
+const aConstant: i32 = 3;
+
+let aStartFunctionLocal: i32 = 2;
+for (var anotherStartFunctionLocal: i32 = 0; anotherStartFunctionLocal < 1; ++anotherStartFunctionLocal);
+for (let aGlobal /* that shadows */: i32 = 0; aGlobal < 1; ++aGlobal)
+  aGlobal;
+
+{
+  let /* actually not */ aConstant: i64 = 5;
+  {
+    let /* still not */ aConstant: f32 = 10;
+  }
+}