Handle local variable shadowing.

test/variable/local_collide_with_function_parameter.wren

@@ -0,0 +1,3 @@
+fn(a) {
+  var a = "oops" // expect error
+}

test/variable/local_in_nested_block.wren

@@ -0,0 +1,6 @@
+{
+  var a = "outer"
+  {
+    io.write(a) // expect: outer
+  }
+}

test/variable/shadow_global.wren

@@ -0,0 +1,6 @@
+var a = "global"
+{
+  var a = "shadow"
+  io.write(a) // expect: shadow
+}
+io.write(a) // expect: global

test/variable/shadow_local.wren

@@ -0,0 +1,8 @@
+{
+  var a = "local"
+  {
+    var a = "shadow"
+    io.write(a) // expect: shadow
+  }
+  io.write(a) // expect: local
+}