Crank: Add header files checks to `export` in Crank

[pyjarrett]

Purpose

Perform checks during header file export to ensure header files can be properly used.

Method

Crank should perform checks of header files contents when the export command is used:

C++ interoperability

Verfiy usage of this block:

#ifdef __cplusplus
extern "C" {
#endif

Header guard (or pragma)

First lines should be:

#pragma once

#ifndef <FILE>_H
#define <FILE>_H

Last line should be:

#endif /* <FILE>_H */

File export visibility

See #1