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_tuple_map.carbon

@@ -0,0 +1,13 @@
+// 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 map[T:! Type](f: fnty (T) -> T, tuple: (T, T)) -> (T, T) {
+  return (f(tuple[0]), f(tuple[1]));
+}
+
+fn inc(x: Int) -> Int { return x + 1; }
+
+fn main() -> Int {
+  return map(inc, (0, 2))[0];
+}