Move ExecProgram to interpreter/ (#802)

Seems to make sense since it's the only interpreter/ dependency in syntax/ right now, so this feels like untangling deps.

Fixes #371

executable_semantics/interpreter/exec_program.h

@@ -0,0 +1,24 @@
+// 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
+
+// Helpers should be added here when logic in syntax.ypp is more than a single
+// statement. The intent is to minimize the amount of C++ in the .ypp file, to
+// improve ease of maintenance.
+
+#ifndef EXECUTABLE_SEMANTICS_INTERPRETER_EXEC_PROGRAM_H_
+#define EXECUTABLE_SEMANTICS_INTERPRETER_EXEC_PROGRAM_H_
+
+#include <list>
+
+#include "executable_semantics/ast/declaration.h"
+#include "executable_semantics/common/ptr.h"
+
+namespace Carbon {
+
+// Runs the top-level declaration list.
+void ExecProgram(std::list<Ptr<const Declaration>> fs);
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_INTERPRETER_EXEC_PROGRAM_H_