Pointers (#1060)

* Naive pointer implementation

* Bug fixes and better pointer tests

* Remove debug print statement

* Implement changes suggested by @geoffromer

Also add a failing test case that tests applying the address-of operator
to an rvalue.

executable_semantics/testdata/pointer/basic.carbon

@@ -0,0 +1,24 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{executable_semantics} --trace %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+// CHECK: result: 0
+
+package ExecutableSemanticsTest api;
+
+fn Main() -> i32 {
+  var x: i32 = 5;
+  x = 10; // changes x to 10
+  var p: i32* = &x;
+  *p = 7; // changes x to 7
+  var q: i32* = &*p;
+  *q = 0; // changes x to 0
+  var y: i32 = *p;
+
+  return y;
+}