Add 3 '*' operators: one prefix, one infix, and one postfix, per #523. (#582)

The presence or absence of whitespace is used to determine which
operator is in use, following the rules described in #520.

Support for prefix * dereference operator follows #523.

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

executable_semantics/testdata/star.carbon

@@ -0,0 +1,16 @@
+// 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
+
+// Never actually called, so this only tests typechecking and semantic analysis.
+fn F(n: Int, p: Int*, q: Int***) -> Int* {
+  var a: Int = n * *p;
+  var b: Int = a*n;
+  *p = b*(*p);
+  **q = p;
+  return **q;
+}
+
+fn main() -> Int {
+  return 0;
+}

executable_semantics/testdata/star.golden

@@ -0,0 +1 @@
+result: 0