Initial implementation of unformed state for local variables. (#1387)

Allows unformed state for local variables. Reports a run-time error when an unformed local variable is used.
- Added declaration without initialization for local variables in the parser.
- Made the init expression of VariableDefinition optional.
- Expanded (alive, dead) to (uninitialized, alive, dead) in the Heap.

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

explorer/ast/statement.cpp

@@ -53,7 +53,11 @@ void Statement::PrintDepth(int depth, llvm::raw_ostream& out) const {
       if (var.is_returned()) {
         out << "returned ";
       }
-      out << "var " << var.pattern() << " = " << var.init() << ";";
+      out << "var " << var.pattern();
+      if (var.has_init()) {
+        out << " = " << var.init();
+      }
+      out << ";";
       break;
     }
     case StatementKind::ExpressionStatement: