Add a public c_function accessible from outside this compilation unit

Property definitions

nitc $ CCompilationUnit :: add_exported_function
	# Add a public `c_function` accessible from outside this compilation unit
	fun add_exported_function(c_function: CFunction)
	do
		body_decl.add "{c_function.signature};\n"
		body_impl.add "\n"
		body_impl.add c_function.to_writer
	end
src/c_tools.nit:58,2--64,4