Switch prelude to a file and refactor lit testing to pass commands directly. (#966)

executable_semantics/data/prelude.carbon

@@ -0,0 +1,11 @@
+// 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
+
+package Carbon api;
+
+// Note that Print is experimental, and not part of an accepted proposal, but
+// is included here for printing state in tests.
+fn Print(format_str: String) {
+  __intrinsic_print(format_str);
+}