Use features new to RGBDS 0.9.2 (#1180)

macros/asserts.asm

@@ -38,7 +38,7 @@ MACRO list_start
 ENDM
 
 MACRO li
-	assert !STRIN(\1, "@"), STRCAT("String terminator \"@\" in list entry: ", \1)
+	assert STRFIND(\1, "@") == -1, STRCAT("String terminator \"@\" in list entry: ", \1)
 	db \1, "@"
 	DEF list_index += 1
 ENDM

macros/scripts/events.asm

@@ -187,7 +187,7 @@ ENDM
 
 	const loadvar_command ; $1e
 MACRO loadvar
-	if STRIN("\1", "VAR_") != 1
+	if STRFIND("\1", "VAR_") != 0
 	; LEGACY: Support for the old name of "loadmem"
 		loadmem \1, \2
 	else