improved checking for liveness when reading and writing memory (#448)

* improved checking for liveness when reading and writing memory

* moving some functions to be methods of State

* finished moving functions into State

* Update executable_semantics/interpreter/interpreter.h

Co-authored-by: Geoff Romer <gromer@google.com>

* moved some comments, other minor edits

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/testdata/experimental_continuation9.6c

@@ -0,0 +1,21 @@
+// 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
+
+// Test that the read from x triggers an error because x is dead.
+// This test also demonstrates how by-reference free-variable capture
+// is dangerous and can happen inside continuations.
+
+fn capture() -> __Continuation {
+  var Int: x = 1;
+  __continuation k {
+    var Int: y = x;
+  }
+  return k;
+}
+
+fn main() -> Int {
+  var __Continuation: k = capture();
+  __run k; // error, lifetime of x is over
+  return 0;
+}

executable_semantics/testdata/experimental_continuation9.golden

@@ -0,0 +1,2 @@
+12: undefined behavior: access to dead value 1
+EXIT CODE: 255