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/syntax/lexer.lpp

@@ -38,6 +38,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 %s AFTER_OPERAND
 
 /* table-begin */
+AMPERSAND            "&"
 AND                  "and"
 API                  "api"
 ARROW                "->"
@@ -134,6 +135,7 @@ string_literal        \"([^\\\"\n\v\f\r]|\\.)*\"
 %}
 
  /* table-begin */
+{AMPERSAND}           { return SIMPLE_TOKEN(AMPERSAND);           }
 {AND}                 { return SIMPLE_TOKEN(AND);                 }
 {API}                 { return SIMPLE_TOKEN(API);                 }
 {ARROW}               { return SIMPLE_TOKEN(ARROW);               }

executable_semantics/syntax/parser.ypp

@@ -141,6 +141,7 @@
 %token
   // Most tokens have their spelling defined in lexer.lpp.
   // table-begin
+  AMPERSAND
   AND
   API
   ARROW
@@ -214,7 +215,7 @@
 %nonassoc EQUAL_EQUAL
 %left PLUS MINUS
 %left BINARY_STAR
-%precedence NOT UNARY_MINUS PREFIX_STAR
+%precedence NOT UNARY_MINUS PREFIX_STAR AMPERSAND
 // We need to give the `UNARY_STAR` token a precedence, rather than overriding
 // the precedence of the `expression UNARY_STAR` rule below, because bison
 // compares the precedence of the final token (for a shift) to the precedence
@@ -367,6 +368,12 @@ expression:
           context.source_loc(), Operator::Deref,
           std::vector<Nonnull<Expression*>>({$2}));
     }
+| AMPERSAND expression
+    {
+      $$ = arena->New<PrimitiveOperatorExpression>(
+          context.source_loc(), Operator::AddressOf,
+          std::vector<Nonnull<Expression*>>({$2}));
+    }
 | expression tuple
     { $$ = arena->New<CallExpression>(context.source_loc(), $1, $2); }
 | expression POSTFIX_STAR