Add syntax for package and library (#792)

Doesn't add much logic, only takes advantage of parser structure for the ordering enforcement.

Note import_nonexistent tests should probably fail, but writing import tests needs a chain of functionality, and I figured I'd just start adding some to validate the syntax (not adding existent imports because that'd require multi-file structure).

executable_semantics/interpreter/exec_program.h

@@ -11,13 +11,12 @@
 
 #include <list>
 
-#include "executable_semantics/ast/declaration.h"
-#include "executable_semantics/common/ptr.h"
+#include "executable_semantics/ast/ast.h"
 
 namespace Carbon {
 
 // Runs the top-level declaration list.
-void ExecProgram(std::list<Ptr<const Declaration>> fs);
+void ExecProgram(AST ast);
 
 }  // namespace Carbon