Use a patch_AddAssertion function instead of extern assertions

include/link/patch.hpp

@@ -3,6 +3,23 @@
 #ifndef RGBDS_LINK_PATCH_HPP
 #define RGBDS_LINK_PATCH_HPP
 
+#include <string>
+#include <vector>
+
+#include "link/section.hpp"
+
+struct Symbol;
+
+struct Assertion {
+	Patch patch; // Also used for its `.type`
+	std::string message;
+	// This would be redundant with `patch.pcSection->fileSymbols`, but `section` is sometimes
+	// `nullptr`!
+	std::vector<Symbol> *fileSymbols;
+};
+
+Assertion &patch_AddAssertion();
+
 // Checks all assertions
 void patch_CheckAssertions();