Generic functions, first baby step (#658)

* generic functions: progress on parser and AST

* finished first baby step

* revisions based on reviews

* Update executable_semantics/interpreter/interpreter.cpp

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

* Symbol => VariableType

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/testdata/generic_function_apply.carbon

@@ -0,0 +1,19 @@
+// 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
+
+fn apply[T:! Type, U:! Type](f: fnty (T) -> U, x: T) -> U {
+  return f(x);
+}
+
+fn positive(x: Bool) -> Int {
+  if (x) {
+    return 2;
+  } else {
+    return -2;
+  }
+}
+
+fn main() -> Int {
+  return apply(positive, false);
+}