Add assertions

Closes #292

include/asm/output.h

@@ -20,6 +20,8 @@ extern struct Section *pSectionList, *pCurrentSection;
 
 void out_SetFileName(char *s);
 void out_CreatePatch(uint32_t type, struct Expression const *expr);
+bool out_CreateAssert(enum AssertionType type, struct Expression const *expr,
+		      char const *message);
 void out_WriteObject(void);
 
 #endif /* RGBDS_ASM_OUTPUT_H */

include/asm/warning.h

@@ -22,6 +22,7 @@ enum WarningID {
 	WARNING_OBSOLETE,
 	WARNING_SHIFT,
 	WARNING_USER,
+	WARNING_ASSERT,
 	WARNING_SHIFT_AMOUNT,
 	WARNING_TRUNCATION,