Generate correct code for locals.

test/duplicate_local.wren

@@ -0,0 +1,6 @@
+class Foo {
+  bar {
+    var a = "value"
+    var a = "other" // expect error
+  }
+}

test/duplicate_parameter.wren

@@ -0,0 +1,6 @@
+class Foo {
+  bar(arg,
+      arg) { // expect error
+    "body"
+  }
+}

test/local_collide_with_parameter.wren

@@ -0,0 +1,5 @@
+class Foo {
+  bar(a) {
+    var a = "oops" // expect error
+  }
+}

test/local_in_middle_of_block.wren

@@ -0,0 +1,14 @@
+class Foo {
+  bar {
+    var a = "a"
+    io.write(a) // expect: a
+    var b = a + " b"
+    io.write(b) // expect: a b
+    var c = a + " c"
+    io.write(c) // expect: a c
+    var d = b + " d"
+    io.write(d) // expect: a b d
+  }
+}
+
+Foo.new.bar